This paper studied the influence of corn straw cover on the processes of runoff and sediment yield in the sloping farmland of a Karst area by indoor simulated rainfall experiments. The treatments were divided into two rain intensities (63 mm/h and 100 mm/h), and five levels of straw cover (0, 20%, 50%, 80% and 100%). The results showed that under the moderate rainfall intensity (63 mm/h), corn straw mulching reduced surface runoff coefficient and surface runoff rate, but increased subsurface runoff coefficient and surface runoff rate. When the straw mulching percentage was greater than 50%, straw cover reduced the amount of soil loss. Under high rainfall intensity (100 mm/h), subsurface runoff was the main component of total runoff on the bare slope, when the bare slope was covered with corn straw, subsurface runoff coefficient and runoff rate were decreased, and the amount of soil loss increased with the increase of surface runoff amount and rate.