AbstractStraw mulching is a typical soil and water conservation measure for slope farms, and it plays an important role in preventing soil erosion. The objective of this study was to quantify the effects of the length and coverage of wheat straw on infiltration, runoff and soil loss. In this study, an indoor simulated rainfall experiment was used to study the characteristics of infiltration, runoff and sediment yield under different straw lengths and coverage when the rainfall intensity was 90 mm h−1. The results show that the straw mulch contributes more to the effect of infiltration, runoff and sediment yield than straw length. Increasing the straw coverage increases the average infiltration rate and reduces the cumulative runoff and sediment yield. An increase in straw length, however, reduces the average infiltration rate of the slope and increases the cumulative runoff and sediment yield. The benefits of straw mulch in increasing average infiltration and reducing runoff and sediment yield are consistent with the law of diminishing marginal returns. The marginal benefit of straw is optimal when the straw cover is 3.5 t·hm−2. The sum of straw mulch and length multiplied by cover divided by length has a power function relationship with slope infiltration, while runoff and sediment yield have a power function relationship with its reciprocal. The average infiltration rate of the slope surface is significantly linearly related to the cumulative sediment yield. The simulation accuracy of the Horton model is higher than that of the Kostiakov model. In this study, when the straw length was 3–5 cm and the mulch rate was 4.5 t·hm−2, the average slope infiltration was highest, and the runoff and sediment yield were lowest. Wheat straw surface mulch, when applied as an effective measure for controlling soil erosion and maintaining soil and water resources on sloped fields, can effectively increase infiltration and reduce runoff and sediment yield; wheat mulch is thus critical for the sustainable development of agriculture and ecology on sloping land.