Abstract: It is of great significance to assess the effective precipitation for agricultural production planning. The comparison between evapotranspiration and precipitation in a certain period to determine the effective precipitation is a simple and universal assessment method in practice, the reasonable statistical time scale parameters is the key to its application. Taking Hejian City, Hebei Province as a case, we conducted the non-linear regression analysis of the effective precipitation and agricultural irrigation in different statistical time scales from 2011 to 2015 based on the reciprocal relationship between the effective precipitation and agricultural irrigation. And through the analysis of the goodness of fit, the optimal range and optimal value of the statistical time scale are obtained. The results manifest that the relationship between the effective precipitation and agricultural irrigation is in power function as the optimal fitting; 7~12 days statistical time scale is the better fitting range and the coefficients of determination are above 0.7; statistical time scale under 10-days reaches the best fitting effect, and the coefficient of determination is about 0.9. In the study process, the high-resolution remote sensing image interpretation data was used to check the statistical data of the staple crop area and the "electric-water conversion" method was used to evaluate agricultural irrigation volume of Hejian City, which effectively improved the reliability of the research.