Abstract: By the way of micro-plot field experimentation and theoretical analysis, micro-relief characteristic and runoff formation under the conditions of site preparation by reverse-slope terrace were studied. The results showed that site preparation by reverse-slope terrace can enlarge the degree of slop where runoff was generated. The range of slope gradient increased was in direct proportion to the width of terrace, which was 2.5°～11.5° per meter, and in the inverse ratio of the slope length, which was 0.5°～7.0° per meter. The length of the slope in runoff area had obvious influence on the total runoff, but the gradient of the slope in the runoff area had the greatest influence not only on the total runoff, but also on the modulus of runoff. Therefore, the regulative mechanism of site preparation by reverse-slope terrace to runoff formation demonstrated shortening the length, increasing the gradient of the slope and changing the form of the slope in the runoff area, which made apparently different formation of runoff and erosion from that of the original slope. So the site preparation work designed according to traditional records of runoff plot with straight slope cannot reach the design standard desired.