Stocking density determines the welfare, growth performance, and even survival of aquatic organisms in aquaculture. However, how stocking density affects cephalopods are still unclear. Here we selected Amphioctopus fangsiao, a promising cephalopod for mariculture, to examine the effect of stocking density on survival, growth, physiology and behavior. Juvenile A. fangsiao (≈ two months post-hatch, 2.41 g ± 0.03) were reared for 21 days under different stocking density groups: 30, 45, 60 and 75 individual/m² (ind./m²) (G1, G2, G3 and G4, respectively). Subsequently, the growth performance, aggressive acts, survival rate, stress response, oxidative stress and glucolipid metabolism were analyzed. This study showed that the high stocking density had a negative impact on growth performance. A. fangsiao reared in G3 and G4 had a significantly lower final body weight, weight gain and specific growth rate than those reared in G1 and G2 (p < .05), and hepatosomatic index in different treatment groups have a similar trend of difference (p < .05). Increased stocking density also caused more fights of A. fangsiao, resulting in a high mortality rate for farmed individuals. The individual numbers of released ink after touch and the recovery time of the breathing rate after air emersion showed that the low stocking density lowered the basal stress level and reduced the recovery time after acute stress of juvenile A. fangsiao. In addition, this study demonstrated that high stocking density has caused oxidative stress and lipid disturbance of A. fangsiao. There were significant differences in reactive oxygen species (ROS), lipid peroxidation's level (MDA content), the activities of superoxide dismutase (SOD) and catalase (CAT), pyruvic acid level (Pyr), total cholesterol level (T-CHO) and triglyceride (TG) between the highest density group (G4) and the lowest density group (G1). Overall, high stocking density (G3 and G4) had a negative effect on reared A. fangsiao. However, there were no significant differences between G1 and G2. Therefore, to improve the welfare of reared A. fangsiao and maximize the economic benefits for producers, juveniles are recommended to rear at a medium density (about 45 ind./m²).