Soil erosion is a serious and widespread problem in China. In this paper we investigate the quantitative relationship between rainfall factors and soil loss in the mountainous area of southwestern China, based on the data of a field experiment. We find that single rainfall factor such as rainfall amount or rainfall intensity can not be generally accepted as the best index for soil loss. We propose a new index which is a combination of different rainfall parameters. The correlation analysis shows this index is highly correlated with soil loss.