Abstract: The paper proposes a method of selecting control areas. It is done by filtering out random fluctuations with daily mean precipitation,as well as by locating natural weather periods with fivee-day running mean and conducting a second filtering for the data. In the meantime,it is proved that the daily mean precipitation obtained through the second smoothing is of normal distribution,being consistent with the principles of elimatology and mathematic statistics in the regression analysis and statistic verification. The method is applicable in the effect-testing and profit-evaluation of showery rainfall in central and souther Human province. The system works on VB5.0 and runs in windors 95/98 environment.