Based on the observational data of monthly maximum temperature, minimum temperature, precipitation, wind speed, relative humidity and sunshine duration of 6 stations in the Northern Tibet from 1961 to 2006, the potential evapotranspiration were computed using Penman-Monteith equation. The linear trend, inter-decadal variation and the seasonal difference of the surface humid index were analyzed, and the relationship between surface humidity index and elements such as sunshine duration, mean wind speed and diurnal temperature range were discussed. The results show that: (1) The annual surface humidity index increased during the period of 1961-2006, and the increase rate was around 0.01-0.05/10a. Also, the seasonal surface humidity index increased in most parts of the Northern Tibet, especially in spring and summer. In recent 26 years (1981-2006), the annual and seasonal potential evapotranspiration showed a declining tendency, and precipitation plays an obvious increasing trend, which resulted in the rise range of annual and seasonal surface humidity index increase, especially in summer. (2) In terms of inter-decadal variations for surface humidity index, from the early to mid 1960s, the climate showed higher wet and lower temperature in the Northern Tibet. While from the late 1960s to the mid 1980s, the climate presented colder and drier. After the early 1990s, the increasing trend of mean temperature existed, the surface humidity index increased significantly, and the climate showed warm and wetter. (3) The surface humidity index was most sensitive to precipitation and relative humidity as well as daily temperature range change, which was more sensitive to sunshine duration and wind speed change.