According to the practical stocking density of Sinonovacula constricta from the experiment ponds in Donggang, Liaoning Province, stocking density was set up at 0, 30, 60, 90 or 120 ind·m^-2. Effect of S. constricta stocking density on water quality in S. constricta culture system in the laboratory was studied by experimental ecology methods. The results indicated that the stocking density of S. constricta had significant effects on the concentrations of DO, NO₃^--N, NO₂^--N, PO₄^3--P and TP (P<0.01) but not NH₄⁺-N (P>0.05). The concentrations of DO, NO₃^--N and NO₂^--N decreased with the increase of the stocking density, while PO₄^3--P and TP concentrations increased with the increase of the stocking density. The higher density of S. constricta (90 and 120 ind·m^-2) improved the cycle and regeneration of P in sediment and the utilization of N in water, which could help to control the concentrations of NO₂^--N and NH₄⁺-N in the culture period. Based on this study, the stocking density of S. constricta ranging from 90 to 120 ind·m^-2 is feasible in the culture ponds, but we need pay more attention on water quality management during the culture period.