Industrial chitosan is primarily derived from shrimp and crab shells. The expanded use of chitosan can help promote the recycling of shrimp and crab shell waste while advancing green biotechnology. Chitosan and its derivatives exhibit a wide range of biological activities, including promoting coagulation, tissue repair and regeneration, antibacterial, anticancer, antioxidant, and absorption properties. Despite its excellent characteristics, chitosan also has limitations, such as poor solubility and weak mechanical properties. The development of nanotechnology provides a foundation for broadening chitosan's applications. Chitosan-based nanocomposites are formed by introducing nanoscale inorganic or organic substances into chitosan, which serves as the matrix. In recent years, chitosan-based nanocomposites have become a research focus in various fields due to their biocompatibility, degradability, and multifunctionality. In medical area, chitosan nanocomposites can promote wound healing by enhancing epithelialization and collagen deposition in the dermis, and they are also promising candidates for bone and cartilage regeneration. Furthermore, chitosan nanocomposites can deliver encapsulated drugs through various pathways, with their nanoscale structure significantly improving the bioavailability and targeting of the drugs. In wastewater and pollutant treatment, chitosan and its derivatives possess strong heavy metal adsorption capabilities, owing to their multifunctional chemical groups, high hydrophilicity, high chemical reactivity, and flexible polymer structure. Chitosan nanocomposites can further enhance these properties, improving their mechanical strength, stability, reusability, and adsorption capacity. In agriculture, chitosan nanocomposites are used as functional components in plant growth inducers, pesticide carriers, fertilizers, growth regulators, and stress inhibitors, thanks to their enhanced antimicrobial properties, targeting ability, and controlled release features. In food industry, the antimicrobial, mechanical, and barrier properties of films and coatings are improved by incorporating nanomaterials into chitosan, which enhances food quality and extends shelf life. In the fisheries industry, chitosan nanocomposites serve as carriers, encapsulants, and immobilizers for bioactive ingredients, enabling the oral delivery of drugs, vitamins, nutrients, genes, and vaccines in the gastrointestinal tract of fish. This paper systematically reviews the research progress and application potential of chitosan-based nanocomposites across the fields of medicine, agriculture, wastewater treatment, food, and fisheries, aiming to provide a comprehensive reference and inspiration for expanding chitosan's applications.