The summer monsoon transition region is a sensitive region of weather and climate. With global warming, its special climatic environmental response has drawn more attention. Nanchang station, Dingxi station, and Ürümqi station are representative of the summer monsoon region, the summer monsoon transition region, and the non-monsoon region, respectively. Temperature, sunshine duration, relative humidity, precipitation, low cloud cover, and wind speed were compared between the summer monsoon transition region and other regions of China over the past 50 years. Trends in pan evaporation for each changing meteorological factor were analyzed. The authors found that, in the summer monsoon transition region, any single meteorological factor change would lead to increasing trends in pan evaporation. However, in the other regions, only air temperature change led to increasing pan evaporation; changes in other factors caused a decrease in pan evaporation. The contribution rate intuitively reflected the effect of each meteorological factor. Results showed that temperature had the greatest contribution to pan evaporation in the summer monsoon transition region with a contribution rate of 48.93%. Wind speed had the greatest contribution to pan evaporation in the summer monsoon region, with a contribution rate of 51.54%. Precipitation contributed the most to pan evaporation in the non-monsoon region, with a contribution rate of 58.57%. In addition, the contribution of low cloud cover reached more than 20% regardless of the region. Therefore, the main factors affecting pan evaporation were different for each different region, while the effect of low cloud cover was significant for all regions.