Based on the daily data of 18 meteorological stations in Hainan Island from 1971 to 2010, spatiotemporal change characteristics of ET_0 of year and the four seasons were analyzed with linear regression and ArcGIS spatial interpolation techniques after calculation with the Penman-Monteith model. Then the sensitivity coefficients were combined with the relative changes of meteorological factors to study the causes of change of ET_0 of year and the four seasons. The results showed that the average annual ET_0 of 18 stations was 1191.4mm, and except for summer, ET_0 of year and the other 3 seasons increased from the northeast to southwest approximately. The average climate change rate of annual ET_0 of 18 stations was -5.0mm*10y-1 in recent 40 years, among which 13 stations' climate change rates were negative and 5 stations' were positive. The average climate change rate of four seasons' ET_0 were -3.1, 1.8, -0.7 and -2.8mm*10y-1, respectively. Overall, 13 stations' annual ET_0 decreased mainly due to the decrease of ET_0 of spring, and the rest 5 stations' increase was mainly due to the increase of ET_0 of summer. The attribution analysis showed that mean wind speed' decrease and sunshine hours' reduction was by and large the main reason for the decrease of ET_0 of year, spring, summer, and autumn in most areas of Hainan Island. Besides the reason above, the decrease of vapor pressure was also one of the major causes for the decrease of winter's ET_0. On the other hand, the major reason for the increase of annual and four seasons' ET_0 was the rise of the average maximum and minimum temperature.