[Objective]To investigate the effects of different straw returning methods on runoff, sediment yield, and nitrogen-phosphorus losses in purple soil sloping farmland of southern Sichuan. [Methods] Runoff plots were established on typical purple soil sloping farmland with a slope of 15°. Five treatments were established: no straw mulch (CK), low straw mulch (T1: 3744 kg/hm2), high straw mulch (T2: 7488 kg/hm2), low straw mulch + decomposing agent (T3: 3744 kg/hm2 + 1.5 kg/hm2), and high straw mulch + decomposing agent (T4: 7488 kg/hm2 + 3 kg/hm2). Runoff, sediment yield, and nutrient loss were monitored under rainfall events. [Results] (1) Results showed that T3 had the best soil conservation effect, significantly reducing runoff depth (by 40.2%) and sediment loss (by 55.3%) while effectively mitigating nitrogen and phosphorus loss. (2) Correlation analysis revealed that nutrient loss was mainly controlled by runoff (p<0.01). However, the weakening nitrogen loss control effect of straw returning over time was attributed to increasing TN concentrations in runoff. (3) all treatments had TN/TP ratios below 22, indicating nitrogen limitation in this region. [Conclusion] Appropriate straw mulching combined with decomposing agent application effectively enhances soil-water conservation and nutrient utilization in purple soil sloping farmland of southern Sichuan, providing scientific basis for optimizing agricultural management practices.