Abstract: Based on the serial meteorological data from Changpin Meteo rological Stations located in the suburb of Beijing the every ten-day period average potential evapot ranspiration(ET_p)were calculated and analyzed.The analysis shows that the ET_p changes in a strong seasonal trend of which the mean and STD fit to the two-harmonics of Fourier series approximately.Moreover,ET_p is a random variable which follows the normal distribution with mean and deviation changing with season.Finally,the probability density function of ET_p is given.