A satisfied method for calculating sediment reduction bencfit in tomprehensive control of small watershed is not available at present.The difference of sediment delivery modulus before and after control is usually used to estimate the benefit,This paper pointed out that the sediment delivery modulus had a great variance as it's not reduced by synthetic control but by precipitation, and there was no Significated interrelation between sediment delivery modulus and characteristic value of precipitation etc, which brought about difficulty of benefit estimating,The author found that there was a closed relationship between Sediment delivery modulus and runoff rate,and the effect of preciptation on sediment modulus could be estimated by runoff rate, so that the sediment reduction benefit might be calculated accurately.