The large deviation of engineering accumulations erosion prediction by the model of USLE is due to the unit plot and the soil erodibility factor cannot be directly used to erosion prediction of engineering accumulations . This study aims to define the gradient and slope length of engineering accumulations unit plot according to the survey of engineering accumulations from six water erosion area in China, analysis the improvement method of engineering accumulations erodibility factor according to relative research achievements and prove the feasibility of this improvement method by published data. Survey results show that the gradient, slope length and surface composition of engineering accumulations from six water erosion area is large different from the unit plot of the model of USLE. So, we choose the mode 35° of gradient and the mean 5m of slop length of engineering accumulations from six water erosion area as the gradient and the slop length of engineering accumulations unit plot. In order to distinguish soil erodibility factor K from engineering accumulations erodibility factor, we give engineering accumulations erodibility factor a name of soil and rock factor (Tr). Bring the index of unit volume soil-rock mixture"s total surface area of rock (Cs) into soil and rock factor, building a function of Tr with soil erodibility factor K. Verification results show that this improvement method of engineering accumulations erodibility factor is reasonable. In conclusion, the defining of the gradient and slope length of engineering accumulations unit plot and the improvement method of engineering accumulations erodibility factor is significant to enhance the accuracy of engineering accumulations erosion prediction by the model of USLE.