Erosivity of rainfall(R)is the potential ability which cause soil erosion and is the function of the physics characteristic of rainfall.There are many methods to calculate the value R, But the calculating process is overlaborate.Based on the observation data from the normal runoff plots(MP I,blank controlled)on the cultivated landof our institute during 1983-1991,the relationships between the different compound parameter of index R andamount of soil erosion has been systematically analysed by the method of correlation analysis in grey system theorysnd computer technique.And based on the regression analysis,the simplified calculation method of the total kineticenergy of once rainfall(especially the torrential rain during a short period of time)has been approached beneficially,and a simplified caculation has been deduced.