The authors estimated the changes of potential evaporation in northern China over the past 50 years (1950-2000) by using the Penman-Monteith model and also analyzed the observation pan evaporation data in the same regions in this paper.The results showed that the potential evaporation and pan evaporation amount had a same decreasing tendency in recent 50 years.The main reasons for the decrease of potential evaporation and pan evaporation amount were due to the decrease of sunshine percentage and wind speed and increase of air humidity during this period of time.From the viewpoint of the energy,the solar radiation reduction was also one of the main reasons for evaporation decrease.The present research indicated that the air humidity increase and the increase in energy resources consumption owing to anthropogenic activities were the two important factors affecting total solar radiation reduction.According to the estimation by Penman-Monteith model the decreased amount of potential evaporation induced by reduction of solar radiation reaching the earths surface accounted for 78% of the total amount of evaporation decreased.