In the summer of 2020, Chongqing experienced the third most precipitation amount in the same period since 1961 and the maximum in the same period in June-July, so it was extremely anomalous. In this paper, we investigate the main causes for the abnormal summer precipitation in Chongqing in 2020 by using daily precipitation from 34 meteorological stations in Chongqing and daily geopotential height, wind, relative humidity and other reanalysis data from NCEP/NCAR as well as monthly sea surface temperature (SST) from NOAA during the summer of 1961-2020. We adopt the modern statistical diagnostic methods such as correlation and synthesis for the analysis. The results show that in summer 2020, especially in June-July, the atmospheric circulation in the high and low latitudes over the Eurasian region showed the “+ - +” circulation type distribution, and the longitude of the circulation was obvious. The first type of circulation configuration with typical rainy summer in Chongqing appeared, thus causing the abnormally more precipitation in summer 2020. The analysis of the external forcing signal indicates that the positive ENSO event from autumn-winter 2019 to spring 2020 made the certainty probability of Chongqing’s summer precipitation increase. At the same time， the positive equatorial Indian Ocean dipole anomaly in autumn 2019 contributed to the anomalous increase of Chongqing’s summer precipitation in 2020. The effects of the two events were superimposed on each other when they occurred simultaneously, causing the extreme anomalous increase of Chongqing’s summer precipitation. ENSO and Indian Ocean dipole were the important precursor signals affecting Chongqing summer precipitation anomaly, so paying attention to these two signals in advance could provide meaningful references for Chongqing summer precipitation prediction.