The probability statistics methods and the usual weather forecasting methods are combined based on the reguest of the special service, therefore data of the relative humidity from 1981-1990 art given. The probability characteristics of inter-region and the characteristics of synthetic weather forecaasting methods of monthly relative humidity of 24 hours at 14 o'clock are discussed. A specieal service system of relative humidity forecasting of Zhengzhdu is built for specieal service and routine relative humidity forecast.