It is important to grasp the temporal change characteristics of water areas of main reservoirs in Gansu for the construction of comprehensive experimental zone of national ecological security barrier in Gansu Province. Based on the 8-day composited surface reflectance products of MODIS (MOD09) from 2000 to 2014, the water areas of Shuangta, Changma, Hongyashan and Liujiaxia reservoirs in Gansu were monitored by using water index of WI1, which was compared with the visual interpreted water areas of real-time satellite data from Lanzhou Regional Climate Center (LZRCC), and their relationship was established. And then the climatic change characteristics of water areas of the four reservoirs were analyzed. Results show that the water areas of the four reservoirs in Gansu retrieved by WI1 based on MOD09 data had better correlation with the real-time monitoring water areas from LZRCC, and their change trends were almost consistent, which indicated that MOD09 data were able to use the research on the climatic changes of reservoir water areas. The interannual change of water areas of the four reservoirs in Gansu was obvious, and the water areas were largest in spring or autumn, while it was smallest in summer. The annual change trends of water areas of the four reservoirs in different seasons were more complex over the last 10 years. Overall, the water areas of Changma and Shuangta reservoirs had no obvious change trend, while that of Hongyashan and Liujiaxia reservoirs in spring, summer and autumn increased volatility over the last 10 years, which was beneficial to agricultural irrigation in middle and western Gansu.