Abstract: With the meteorological observation of temperature, precipitation, wind speed and so forth at 7 stations in Chamdo city during 1981 -2012, this study investigated the temporal and the spatial distribution of potential evapotranspiration (ET0), the surface humid index and their seasonal difference with its main meteorological influencing factors using Penman - Monteith Model. The results show that the annual ET0is decreasing with a rate of 14. lmm/10a, the largest decreasing rate mostly appears in spring and summer. The mean annual surface humid index is higher in the northern and southeast parts than that in the central part of Chamdo. The index is generally increasing at the rate of 0. 027/10a in Chamdo, and the largest in- creasing rate also appears in spring and summer. The increase of the surface humid index is mainly caused by the significant in- crease of the precipitation and the decrease of wind speed, the daily range of the temperature may also plays an important role in the increase of the index.