Based on the 23 observational stations data of monthly precipitation and mean monthly temperature over Ningxia from 1961 to 2004,by use of the surface humid index computed from the data, decadal variation and the seasonal differences of the surface humid index over Ningxia have been analyzed.The geographical distributions and trends of surface humid index have been given in this area. The results indicated that the total trend of the surface humid index of Ningxia has dropped in past more than 40 years, namely the earths surface is becoming dry. The change of the surface humid index has obvious phase characteristics. Cold and wet climate was main characteristic of Ningxia from the sixties to latter stage of the seventies. The climatic characteristic was cold and dry from 1978 to 1988. And the climate has been warm and dry since the 1990s. The drying trend of Ningxia generally occurred in autumn.The centre area where the earths surface becomes dry notably was in the central arid strip part and the north part of loess hills area of south in Ningxia. The winter has an tendency of wetting in loess hills area of south in Ningxia. But there is no notably wet or dry tendency in all parts of Ningxia in spring and summer.