Based on the daily data of 20cm evaporating pans as well as other meteorological factors of 18 automatic meteorological observation stations from 1966 to 2001 over Hainan Island, changes in pan evaporation and their causes are analyzed by the methods of ArcGIS and the mathematical statistics. The results show that the annual evaporation of Hainan Island showed a fluctuating decline, and the decrease of annual evaporation mainly occurred in spring, winter and summer. The annual evaporation was less in the northeast and greater in the southwest. The large value areas were mainly concentrated in the southwest, while the smaller ones were mainly concentrated in the northeast and middle areas. The MK test confirms that annual evaporation had significantly decreasing trends and the significant mutation year was about 1994. Among the change factors affecting evaporation, the sunshine duration and wind speed were the main factors that caused the decrease of evaporation. The influence of precipitation on the evaporation of Hainan Island was second only to the wind speed and sunshine hours. The temperature was not the main factor for decreasing evaporation, and humidity might be the influence factor of the of Hainan Island decreasing evaporation.