In this paper,soil water erosion of lucerne-spring wheat rotation soil in Dingxi under has been studied.The results showed that Pounding time of lucerne field with till(T1) is the longest.Runoff time of lucerne field with no till(T2) was the longest,which was significantly higher than three treatments.In the whole process of rainfall simulation,the runoff velocity of no till was lower than that of tilled.In four treatments,the runoff velocity of lucerne field with no till(T2) was the lowest,but between two tilled treatments had not significant difference.The infiltration velocity of lucerne field with no till(T2) was higher than that of other three treatments.Straw covering could decreased cumulated runoff and sediment and increased infiltration in the same time.In the different treatments,the result of resist water erosion of lucerne field with no till(T2) and 100% straw cover was obvious.