In a polyculture system，sea urchin（Strongylocentrotus intermedius）juveniles weighing (1.4±0.6) g were stocked into 16 plastic tanks of each 45 cm×31 cm×30 cm at a rate of 11 individuals per tank and polycultured with juvenile sea cucumber (Apostichopus japonicus Selenka) averaging (3.4±0.6) g in body weight at a rate of 0 (Group A), 3 (Group B), 5 (Group C) and 10 individuals (Group D)，and manila clam (Ruditapes philippenarum) averaging (12.4±1.7) g in body weight at a rate of 6 individuals per tank at water temperature of 13.8-20.0 ℃ for 77 days. The sea urchin were ad libtum fed kelp (Laminaria japonica) and the sea cucumber fed marine benthic diatoms cultured by the effluents from the tanks. Survival rates of the sea cucumber and the sea urchin were all found to be almost 100%, no significant differences (P>0.05). However, there were significantly 9.12%, 7.24% and 10.06% higher in specific growth rate (SGR) of the sea urchin in Group B, Group C and Group D than that in Group A (P<0.05), respectively. There was significant lower feed conversion ratio（FCR）of the sea urchin in the polycultured tanks than that in the monocultured tanks (P<0.05), 20.2%，15.5% and 18.0% lower in Group B, Group C and Group D than that in Group A, respectively. The optimal ratio of sea urchin to sea cucumber was 11∶3, with the optimal stocking density of 64 121.1 g·m^-3 sea cucumber. Poor growth and high mortality were observed in manila clam in the polyculture system and in sea lettuce Ulva lactuca cultivated by the tank effluents. Nitzschia sp. and Cocconeis sp. were predominantly found in the benthic algae cultivated by the effluents from the tanks. The sea urchin in the polycultured tanks had significant higher gonadal C18:3n-3 and EPA+DHA levels and n-3/n-6 ratio than those in the monocultured group had. In the polyculture water, ammonia－N levels were registered low and constant, whereas high dissolved oxygen levels were observed in the polyculture system.