Abstract: Based on the daily Meteorological data of AnXin Meteorological Observation Station in Baiyangdian marsh within 1972-2012 years, using trend analysis, standard deviation, Mann-Kendall mutation test and Morlet wavelet analysis method, this article analyzes the potential evapotranspiration variation in Baiyangdian marsh. The analyzation reveals that, in the nearly 41 years, the potential evapotranspiration in Baiyangdian Marshs shows a downward trend, especially when it comes to the inter-annual, spring and autumn. The fluctuation analysis of each season evapotranspiration reveals that the maximum volatility happens in summer, while the minimum in winter. The marsh evapotranspiration sometimes takes mutations reduce processes, like annual potential evapotranspiration decrease in 1988, summer potential evapotranspiration decrease in 2001, fall potential evapotranspiration decrease in 1991. The annual potential evapotranspiration variation takes a 8-10 years cycle, the spring potential evapotranspiration variation a 5-7 years cycle, the summer and autumn a 8 years cycle, and the winter a 10 years cycle.