Abstract: Astronomical factor is one of the important factors in the regional river flow, but different astronomical factors have different degrees of influence and patterns on the runoff of the basin. To investigate the most important astronomical factors affecting the runoff of the Qiantang River Basin and their influence on the runoff of the basin, this paper identifies 26 astronomical factors such as sunspot relative number, lunar declination angle and 24 solar terms by using the methods of linear correlation, mutual information and gray correlation analysis and analyses the variation patterns between the major astronomical factor and the runoff of the basin. The main conclusions are as follows:（1）Based on methods of linear correlation, mutual information and gray correlation analysis, the results of correlation analysis has shown that the lunar declination angle is the most influential astronomical factor on the inflow of the Qiantang River Basin；（2）the runoff of the basin is abundant and the probability of wet year is 46.4% when the lunar declination angle changes from large to small, while the probability of dry year is 54.5%, and it is prone to extreme year when the lunar declination angle changes from large to small；（3）the probability of dry year is about 66.7% and extreme dry year is frequent as well during the peak of lunar declination, whereas the extreme events happens frequently with 50% of the years being extreme during the trough of lunar declination. The results can provide a certain support for future long-term runoff evolution analysis and flood forecasting in the Qiantang River basin.