To explore methods to reduce phosphorus leaching and thus improving river water quality, we conducted field experiments on the effects of reduced-phosphorus fertilizer and combined application of organic and inorganic fertilizers on phosphorus leaching in purple paddy soil with conventional paddy-upland rotation tillage. With these treatments, phosphorus seepage decreased over time, but the amount leached was high and variable in the early stages, gradually reaching stable equilibrium after 55 days. Total dissolved phosphorus (TDP) had the highest levels of phosphorus leaching into the waters of the purple paddy soil. The total phosphorus (TP) and total dissolved phosphorus (TDP) contents in soil leachates were as follows: [optimized fertilizer + pig manure application] > [optimized fertilizer + straw return] > [optimized fertilizer] > [optimized fertilizer with phosphorus reduced by 20% + pig manure application] > [optimized fertilizer with phosphorus reduced by 20% + straw return] > [without phosphate]. TP loss ranged between 0.295 and 0.493 kg/hm². Organic fertilizer promoted phosphorus leaching, and the effect of pig manure was larger than that of rice straw. Decreasing the amount of chemical fertilizer can reduce the amount of phosphorus leached into the soil. To improve agricultural production and control nonpoint source pollution, it is important to reduce the use of chemical fertilizers and use combined application of organic and inorganic fertilizers.