Studies on the stocking density of Japanese seabass (Lateolabrax japonicus) are limited. In addition, most studies have been mainly conducted in small laboratory water bodies, industrial cement ponds, and soil ponds, with few studies on seawater cages. Therefore, A 70-day feeding experiment was conducted to evaluate the effects of different stocking densities on growth performance, digestive enzyme activities, non-specific immunity, and anti-stress ability of L. japonicus [initial weight (220.52±2.50) g]. Four culture density gradients were set: 20 tail/box (M20, 4.43 kg/m³ ), 30 tail/box (M30, 6.61 kg/m³ ), 40 tail/box (M40, 8.82 kg/m³ ), and 50 tail/box (M50, 10.98 kg/m³ ). Each group had three replicates randomly placed in 12 seawater cages (1 m× 1 m× 1.2 m). The results were as follows. The weight growth rate (WGR) and specific growth rate (SGR) reached maximum values in the M40 group (90.33% and 0.92%/d), and no significant difference was observed with the M30 group (P＞0.05), but both were significantly higher than the M20 and M50 groups (P＜0.05). The condition factor (CF) of the M50 group was the lowest (1.72), which was significantly lower than the other three groups (P＜0.05). The CF of the M40 group was the highest (1.84). The highest ether extract content in entire fish and muscle in the M30 group was significantly higher than that in the M50 group (P＜0.05). The lowest ether extract content in muscle in the M50 group was 87.53%, which was significantly lower than that in other groups (P＜0.05). Trypsin activities in the intestinal tract reached the highest value in the M40 group and were significantly higher than in other groups (P＜0.05). Simultaneously, lipase activities had the lowest value in the M50 group and were significantly lower than that of the other groups (P＜0.05). The activities of superoxide dismutase (SOD) of the M40 group were significantly higher than that of the M20 and M50 groups (P＜0.05), but no significant difference was observed with that in the M30 group (P＞0.05). The malondialdehyde (MDA) content in the M50 group was the highest and significantly higher than that of the other three groups (P＜0.05). Meanwhile, the activities of alkaline phosphatase (AKP), lactate dehydrogenase (LDH), and lysozyme (LZM) had a similar tendency with SOD. With increased stocking density, corticosterone (COR) activity in the serum initially decreased and then increased; the activity of the M40 group was the lowest and was significantly lower than that of the other groups (P＜0.05), whereas the activity of the M50 group was the highest and significantly higher than that of the other groups (P＜0.05). The activities of creatine kinase (CK) and adrenocorticotropic hormone (ACTH) showed a similar tendency with COR. In conclusion, under the experimental conditions, the growth performance and non-specific immune ability of L. japonicus were optimal in the M30 and M40 groups, with a stocking density of 6.61–8.82 kg/m³ .