Abstract: Based on 25 observational station's data of monthly maximum temperature, minimum temperature, precipitation, wind speed, relative humidity and sunshine duration over Tibet from 1971 to 2005, the data of the maximum potential evaporation was computed by Penman Monteith Model. The surface humid index is the ratio of rainfall to maximum potential evapotranspiration (ET0). The distribution, inter-annual change and the seasonal difference of ET0 and surface humid index were analyzed, also the impact climatic factors for the change trend of surface humid index were discussed. The main results were as follows: In recent 35 years, the annual precipitation trend increased significantly and the rate of increasing was 15.0 mm/(10 a) in Tibet, the respective decreasing trend of annual ET0 was prevailing and its decrease were -4.6--71.6 mm/(10 a). The decrease of annual surface humid index was insignificant in southwestern Ngari district and Nyalam county, annual surface humid index was increasing at the rate of 0.02-0.09/(10 a) in others district of Tibet. On an average in Tibet, the increasing trend of annual humid index was significant, and its increase was 0.04/(10 a), especially in last 25 years (1981-2005). Also, the increasing trend of seasonal surface humid index was overwhelming, particularly in summer. From the 1970s to the 1980s, the inter-annual characters behaved lower temperature and lower humidity. In the 1990s, the air temperature rose steadily, and surface humid index increased obviously. The phenomena indicated warm and wet climate over Tibet. It is thus obvious that the increase of the surface humid index is mainly caused by the remarkable increasing of precipitation and relative humidity as well as the significant decreasing of the daily temperature range, declining of the sunshine duration and wind speed may also played an important role in increasing of humid index.