The paper analyzed the erosion processes of purple soil slopes using the method of artificially simulated rainfall experiment and lab analysis to study the effects of vegetation coverage on the erosion processes of purple soil slopes, and discussed the critical point problem of slope vegetation cover. The results showed that: (1) As a whole, with the increase of vegetation coverage, the runoff on purple soil slopes decreased. The was a significant negative relationship between the vegetation coverage and the runoff. However, when the vegetation coverage was less than 25%, the changes in runoff were not obvious. When the vegetation coverage reached 75%, the stable value of the runoff was low, being obviously lower than that when the vegetation coverage was 0%, 25%, and 50%. (2) Sediment yield decreased rapidly with the increase of vegetation coverage. There was a significant quadratic term correlation between the vegetation coverage and the runoff. When the vegetation coverage was below the critical value of 25%, the vegetation played an important role in decreasing sediment load. A vegetation coverage 50% was the critical coverage for soil erosion control on purple soil slope. (3) Soil microaggregates (< 0.25mm) were the main parts in eroded sediment. Basically, all levels of soil aggregates decreased with the increase of vegetation coverage. (4) Through statistical analysis, the vegetation coverage caused an extremely significant effect on runoff and sediment yield (p < 0.01). Moreover, when the vegetation coverage reached 50%, vegetation played a vital role in soil and water conservation, and reduced soil and water erosion on purple soil slopes.