In this paper, according to principle of runoff formation and infiltration, the difference of runoff formation between the prototype and the normal mock-up of small drainage was analyzed, and the conception of runoff formation distortion in soil erosion experiment of the mock-up of small drainage was put forward for the first time. Again on the basis of process curve of runoff formation and infiltration obtained by the test of artificial simulation rainfall on typical slope in Liudao gully drainage, the runoff formation distortion coefficient of the normal mock-up of small drainage in various scale was calculated theoretically. It improved applicable value of test of soil erosion with normal mock-up of small drainage. And it exhorted people to transform mock-up to prototype by multipling runoff formation distortion coefficient and tested figures when tested data were applied. Otherwise, the wasting phenomenon of project design in soil and water conservation would be caused.