The purpose of this study was to understand the characteristics of variations in soil surface micro-topography and soil erosion, providing evidence for the effective prevention and control of soil erosion in sloping farmland of purple soil region. Based on the indoor simulated rainfall experiments, the spatial variation of soil surface micro-topography was studied in sloping farmland with cross ridge in purple soil region under the conditions of increasing and decreasing rainfall series. Soil surface roughness was used to indicate the dynamic changes in the soil surface micro-topography. On this basis, soil surface roughness was calculated and DEM was established, and the effect of soil surface micro-topography on runoff and sediment yield was explored. The results showed that the change of soil surface elevation was characterized primarily by the reduction in increasing and decreasing rainfall series, concentrated mainly in the range of -2 to 0 cm. Under the increasing rainfall series, soil surface micro-topography changed obviously in the middle course of rainfall and the rill erosion kept intensifying. However, in the decreasing rainfall series, soil surface micro-topography began to collapse in earlier rainfall course. In the late rainfall course, rills tended to broaden and deepen, but the starting point of rill formation was relatively stable, which did not extend and develop along the ridge direction. In the increasing rainfall series, the variation of soil surface roughness was not obvious in the first two rainfalls, whereas the soil surface roughness showed a significant increase in the third rainfall. The soil surface roughness increased first and then decreased under the decreasing rainfall series. The difference of sediment yield was not significant between the increasing and decreasing rainfall series. The sediment yield in the increasing rainfall series was 3.9% higher than that in the decreasing rainfall series, while the volume of runoff under the condition of the increasing rainfall series was 14.8% lower than that in the decreasing rainfall series. When cross ridge-breaking happened, surface water movement dominated in surface runoff on the ridge-breaking slope. The sediment yield depended on the soil loss of ridge breaking and runoff washing under high-intensity rainfalls in slope farmland of purple soil region. Compared with the decreasing rainfall series, the initial time of runoff and sediment yield was later in the increasing rainfall series. Effect of rainfall conditions on runoff was greater than the effect of erosion yield. Cross ridge could be used to effectively reduce soil erosion in light-intensity rainfalls in slope farmland of purple soil region. However, ridge breaking was likely to happen for the cross ridges of sloping farmland in purple soil region, and it might increase soil erosion when the rainfall intensity increased to 1.5 mm/ min.