Through continuous field observation of soil erosion in the Soil and Water Conservation Station of Suining,processes of soil erosion on purple parent material of the Suining group were studied by collecting the data from 1991 to 2000 and using the method of data analysis.Results showed that annual runoff module from soil erosion plots on the parent material closed for 10 years changed with annual rainfall amount,but on the whole,annual soil erosion module had a tendency to decrease year by year.Runoff depth was positively related to rainfall amount,and inversely related to rainfall duration.A regression equation for the relationship between runoff depth and rainfall amount was established,and the critical rainfall amount that can induce runoff was found to be 15.85 mm.Rainfall amount and mean rainfall intensity did not significantly influence soil erosion amount.During the first 4 years,the regression equation could be formed between soil erosion amount and runoff depth,and the critical runoff depth that can induce a large amount of soil erosion was 9.05 mm.It is evident that the closing management may prevent and cure soil erosion on the purple parent material,and soil erosion gradually changes from severe degree to light degree after closing for at lest 8 years.