Based on natural rainfall erosion characteristic, the paper studied on soil crust formation on the slope farmland according to rainfall, runoff and rainfall runoff slope comprehensive methods in indoor. Some characteristics are found through measuring the crust hardness, bulk density and thickness of crust layer. In the process of soil crust forming, rainfall function is mainly impact, runoff function is mainly depositing and condensing, and their functions increase as rainfall intensity and depth of runoff increasing. But the function is also limited while slope degree increasing, the growth of crust is no good. On the whole, the function of rainfall is more powerful than runoff.