<p>The Sichuan Basin, situated east of the Tibetan Plateau, exhibits robust diurnal variation in precipitation. Moreover, the weather and climate in this region are strongly influenced by the summer monsoon, but how the summer monsoon affects the diurnal variation of local precipitation remains unclear. Therefore, diurnal variation of precipitation in Sichuan Basin before the summer monsoon onset and that in the summer monsoon season are concerned in this study, and the role of summer monsoon in modulating the diurnal variation of precipitation is analyzed. It is found that the easterly and southerly anomalies are crucial to the diurnal variation of precipitation in Sichuan Basin, and the summer monsoon tends to uniformize the peak periods of precipitation in western Sichuan Basin (WSB) and eastern Sichuan Basin (ESB). Before the summer monsoon onset, the easterly and southerly anomalies in WSB and ESB peak at different times, leading to the precipitation maxima appearing in these two subregions at different times. In the summer monsoon season, the flow over Sichuan Basin is featured by strong easterly and southerly anomalies, and peaks during 21 − 02 LT. Correspondingly, the winds during 21 − 02 LT possess the most typical characteristics of monsoon flow, with the strongest easterly and southerly anomalies in both WSB and ESB. Strong easterly and southerly anomalies during 21 − 02 LT contribute to the most abundant water vapor in entire Sichuan Basin, and thereby lead to the subsequent precipitation peaks during 03–08 LT in both subregions.</p>