Based on the location observation data of the wild outflow plots from 2018 to 2019, taking the corn monoculture treatment without straw mulch (M₀) as the control, this study was aimed to explore the effects of maize monocropping treatment and different straw coverage (M₁~M₅, 1 111, 2 222, 3 889, 5 556, 6 944 kg/hm²) on runoff and sediment yield characteristics of sloping farmland in the karst area of Guizhou Province. The results showed that: (1) Under different rainfall levels, straw mulching had better soil and water conservation effect, and with the increase of mulching amount, the total runoff and sediment yield decreased gradually, but there was no significant difference between M₄ and M₅ treatments (p > 0.05); (2) The runoff and sediment yield of each treatment from seedling stage to jointing stage were greater than those of the latter two stages, and the runoff and sediment yield of each mulching treatment in each stage were lower than that of the control; (3) Compared with the control, the annual average runoff depth decreased by 21.85%~50.46%, and soil erosion modulus decreased by 50.10%~85.87%; (4) As the amount of straw mulching and the amount of straw buried in the soil increased, the runoff and sand production of each treatment showed a downward trend. However, when the straw mulching amount exceeded 5 556 kg/hm² and continued to increase to 6 944 kg/hm², the soil and water conservation did not change significantly. Straw mulching was an effective way to control soil and water loss of sloping farmland in karst area, and the coverage amount of about 6 000 kg/hm² was recommended in the soil and water conservation.