Evaporation is an important part of hydrological circulation. The Mann-Kendall's trend test technique was used to detect the trend of pan evaporation in the Haihe River basin (HRB) from 1961 to 2006. The results showed that there is a statistically significant decreasing trend of pan evaporation in the HRB. Correlation analysis, partial correlation analysis, and principal component analysis were used to analyze the relationship between pan evaporation and four meteorological factors such as mean temperature, relative humidity, wind speed and sunshine duration. The paper set up an empirical formula (E-THWS formula) for calculating pan evaporation based on the four meteorological factors, and announced the effect of four meteorological factors in paradox theory of pan evaporation of Haihe River basin . The results indicated that the increasing effect of pan evaporation by mean temperature and relative humidity is less than the decreasing effect of pan evaporation by average wind speed and sunshine duration.The decrease of average wind speed is the chief reason resulting in the decrease of pan evaporation in HRB.