Based on a simulated rainfall experiment in the field, effect of grass coverage on slope erosion was studied, and comparison between runoff erosion power, expressed by runoff depth and peak discharge modulus, and rainfall erosion force by sediment yield was conducted to analyze the role of the vegetation regulating erosion dynamics on the slope. Results show that the vegetation cover affected runoff and sediment generation significantly. Runoff and sediment yield reduced rapidly with the vegetation coverage increasing from 0% to 60%, however, when coverage exceeded a critical value, 80%, the effect was not so significant and leveled off. The critical vegetation coverage ranging from 60% to 80% is determined in this study. Both runoff erosion power and rainfall erosion force were positively related with sediment yield, and the former was more significant than the latter, suggesting that runoff erosion power may be used to better simulate erosion dynamics. The presentation of the effect of vegetation coverage on erosion by the ratio of runoff erosion power/erosion volume reflects existence of critical vegetation coverage, which can be used as a new index to evaluate regulatory effects of vegetation cover on soil erosion dynamics.