A field study was conducted to assess the efficacy of a long-acting slow-release amendment at various application rates in remediating lightly to moderately co-contaminated paddy fields (Cd and As) under flooded conditions. With no amendment as the control (CK), seven amendment rates were evaluated for their effects on Cd and As accumulation in brown rice, soil physicochemical properties, and rice yield. Compared to CK, amendment application significantly reduced Cd and As concentrations in brown rice by an average of 53.4% and 25.1%, respectively, and was accompanied by a 19.7% average increase in grain yield. With increasing amendment rate, grain Cd and As levels declined rapidly at first and then leveled off, whereas grain yield showed a parabolic response, increasing initially and then decreasing. Soil pH, cation exchange capacity, and available phosphorus content increased with the application rate. The optimal application rate for cost-effective remediation was approximately 285.0 kg/hm². Overall, this study identified a promising amendment and determined its optimal application rate, providing a practical strategy for the safe utilization of lightly to moderately Cd-As co-contaminated farmland.