Abstract: This study was conducted to systematically evaluate the effects of different stocking densities of triangle sail mussel (Hyriopsis cumingii) on water quality parameters and the growth performance of co-cultured organisms in polyculture ponds with Chinese mitten crab (Eriocheir sinensis). The experiment was carried out from May to October 2023 at Kaijiang Minsheng Fisheries Development Co., Ltd., with a total culture period of 136 days. Three experimental treatments with varying Hyriopsis cumingii densities (A: 9 000 ind./hm², B: 12 000 ind./hm², and C: 15 000 ind./hm²) were established in ponds stocked with Eriocheir sinensis (12 000 ind./hm²), bighead carp (Aristichthys nobilis, 300 ind./hm²), and silver carp (Hypophthalmichthys molitrix, 250/hm²), while a control group (D) without Hyriopsis cumingii was set up for comparison. During the experimental period, key growth indicators of cultured organisms (body size, survival rate, yield) and water quality parameters were monitored regularly. The results showed that increasing Hyriopsis cumingii stocking density was negatively correlated with the final body size and survival rate of the mussels. Specifically, the low-density group (A) had a significantly lower Hyriopsis cumingii yield than the medium (B) and high-density (C) groups (p<0.05). Regarding Eriocheir sinensis, both male and female individuals in all experimental groups (A, B, C) exhibited significantly smaller average body sizes and lower yields compared to the control group (D) (p<0.05), whereas no significant difference in survival rate of Eriocheir sinensis was observed among all treatments (p>0.05). In contrast, the growth parameters (body weight gain, survival rate, yield) of bighead carp and silver carp remained stable across all experimental and control groups, with no statistically significant differences (p>0.05). In terms of water quality improvement, polyculture of Hyriopsis cumingii significantly enhanced water transparency in all experimental groups compared to the control. The high-density group (C) showed the lowest levels of dissolved oxygen (DO), pH, total dissolved solids (TDS), electrical conductivity (EC), total phosphorus (TP), and phosphate (PO₄³⁻-P). Notably, the medium-density group (B) achieved optimal water quality status, with significantly lower concentrations of ammonium nitrogen (NH⁴⁺-N), total nitrogen (TN), chemical oxygen demand (COD_Mn), nitrite (NO₂⁻-N), and nitrate (NO³⁻-N) compared to the high-density group (C) (p<0.05). Results suggest that a Hyriopsis cumingii stocking density of 12 000 ind./hm² provides optimal conditions, significantly reducing key water quality parameters (NH₄⁺-N, NO₂⁻-N, PO₄³⁻-P, TN, TP, COD_Mn) while maximizing overall production under these experimental conditions.