Based on Landsat (MSS, TM, ETM+)satellite series and Environmental Mitigation satellite data and the 40 years of meteorological data near the Thari Namtso of Tibet, we investigated the area change in the lake and the possible meteorlogical causes. Results show that the lake experienced decrease firstly and then increased 7.08 km² in the area during the period 1975-2011. The glacier retreats, both the temperature and rainfall saw upward trends, but evaporation and maximum depth of permafrost experienced downward trend in the watershed of Thari Namtso during the last 40 years. The causal effect analysis indicated that the area of Thari Namtso has linear relationship with annual mean temperature, that is, the area was positively associated with the annual temperature. Therefore, we infer that the increasing temperature is likely to be the main factor of lake expanding.