Microplastics (MPs), due to their small size and strong concealment, have become a new type of pollutant widely present in the global marine ecosystem. Because of their complex sources and wide distribution in the marine environment, they are easily ingested by aquatic organisms and migrate and accumulate among organisms at different trophic levels along the food chain, posing a potential threat to biological health. Existing investigations have shown that over 400 species of fish in Asia have been confirmed to have microplastics in their bodies, and at least 800 species of fish worldwide have been recorded to have microplastics in their bodies. Microplastic pollution has presented a widespread biological exposure characteristic. Besides causing direct ingestion risks, microplastics, with their large specific surface area, rough surface structure, and strong hydrophobicity, can provide an ideal substrate for the attachment, colonization, and biofilm formation of microorganisms (including potential pathogenic bacteria), thus becoming an important carrier for the migration and spread of pathogenic bacteria in the marine environment. The concept of “Plastisphere” was proposed in 2013, further revealing the unique microbial community structure on the surface of microplastics, and in some samples, up to 24% of the microorganisms belonged to the genus Vibrio. Since then, a large number of studies have detected the extensive attachment of Vibrio bacteria on the surface of microplastics in multiple global sea areas, including the Baltic Sea, the Mediterranean Sea, and the Bohai Bay and Maowei Sea in China. Against this background, an increasing number of studies suggest that microplastics may significantly increase the bioavailability of pathogenic bacteria through the “Trojan horse effect”, promoting the joint entry of microplastics and pathogenic bacteria into organisms and inducing oxidative stress, inhibiting metabolic functions, and weakening the immune defense system of the body. However, most existing experimental studies have used exposure concentrations higher than those in the actual environment, making it difficult to truly reflect the ecological risks of the combined effects of microplastics and pathogenic bacteria in the natural marine environment, and the underlying biological response mechanisms remain unclear. Therefore, this study took the juvenile of the important economic fish Sebastes schlegelii in the Yellow and Bohai Seas as the research object, focusing on the combined toxicity effects of environmental concentration polyethylene microplastics and Vibrio parahaemolyticus on the juvenile of Sebastes schlegelii. The experiment set up a blank control group, a polyethylene microplastic exposure group at 20μg/L, a Vibrio parahaemolyticus exposure group at 10?CFU/mL, and a combined exposure group of polyethylene microplastics and Vibrio parahaemolyticus, and conducted indoor simulated toxicological experiments. The liver tissues of the juvenile Sebastes schlegelii were collected at 0, 7, 14, and 21 days of the experiment to determine the activity of superoxide dismutase (SOD), the content of malondialdehyde (MDA), and the activities of acid phosphatase (ACP) and alkaline phosphatase (AKP); at the same time, the muscle tissues were collected on the 21st day of the experiment to analyze their basic nutritional components, amino acid composition and content, and evaluate the amino acid nutritional value. The results showed that in terms of oxidative stress, the combined exposure of polyethylene microplastics and Vibrio parahaemolyticus significantly reduced the SOD activity in the liver of juvenile Sebastes schlegelii and significantly increased the MDA content (P<0.05), indicating that the oxidative damage caused by the combined stress exceeded the antioxidant capacity of the body, intensifying the lipid peroxidation of liver cells. In terms of immune response, the combined stress initially induced a significant increase in the activities of ACP and AKP (P < 0.05), indicating that the non-specific immune defense system of the body was activated to assist in the clearance of foreign substances and enhance detoxification metabolism. In the later stage of the experiment, the activity of ACP decreased while that of AKP remained high, suggesting that the combined effect of polyethylene microplastics and Vibrio parahaemolyticus could cause chronic damage to the cell membrane structure and function and continuously induce detoxification metabolic processes, thereby posing a long-term potential risk to the immune system. The analysis of muscle nutrient components showed that the combined exposure led to a significant increase in the water and ash content of the body (P < 0.05), and a significant decrease in the content of five essential amino acids, including valine, isoleucine, leucine, phenylalanine and lysine (P < 0.05). Although it did not cause substantial damage to the nutritional quality of muscle protein for the time being, the reduction in the content of multiple essential amino acids has already indicated a potential risk of deterioration in the nutritional value of the muscle. In summary, this study revealed the hepatotoxic mechanism and the potential impact on the nutritional quality of muscle caused by the combined exposure of environmental concentrations of polyethylene microplastics and Vibrio parahaemolyticus on juvenile Sebastes schlegelii. It can provide data support for in-depth assessment of the aquatic ecological risks of the synergistic effects of microplastics and pathogenic bacteria, and offer a scientific basis for formulating targeted strategies for the prevention and control of marine microplastic pollution. It also suggests that in future research, we need to pay close attention to the chronic toxic effects and molecular mechanisms of the combined action of pollutants in the environment under low-dose and long-term exposure.