This paper dwelt on the generating mechanism of various components of runoff, such as the infiltration excess runoff, saturated surface runoff, subsurface runoff, groundwater runoff etc., and analyzed the conditions of thier existence and generation. On the basis of above analysis a Inter-face-runoff-generating law is presented. Further, the key problems in mechanism of runoff-generation in watershed is analyzed. It is good for better understanding whether conceptually or theoretically the runoff generating...