Abstract: Based on the fusion of precipitation data and ERA-5 reanalysis data, the law and physical mechanism of plateau monsoon affecting precipitation in Chengdu in August are studied. The results show that the precipitation in Chengdu in August is characterized by high in the west and low in the east. The four plateau monsoon indexes in Augusts are negatively correlated with the precipitation in Chengdu during the same period, indicating that the plateau monsoon increases (decreases) in August and the precipitation in Chengdu decreases (increases). The negative correlation is the strongest in mid-August, indicating that the plateau monsoon has the most significant impact on precipitation in Chengdu during this period. In the strong TPMI years, the Western Pacific Subtropical High is southerly, the Bay of Bengal trough is stronger easterly, the 500 hPa anomaly field is mainly distributed in the meridional direction of “+ −”, the plateau low pressure is strengthened, and the east of Sichuan is affected by the northerly airflow. The Chengdu area is controlled by the sinking motion and the whole layer of water vapor divergence, the convection is stable, which is not conducive to precipitation. In weak years, the Western Pacific Subtropical High is to the north, the trough over the Bay of Bengal is to the west, the 500 hPa anomaly field is mainly zonal distribution of “− +”, the plateau low pressure is weakened, the eastern part of Sichuan is affected by the southerly airflow, there is obvious water vapor convergence and upward movement, the convection is not stable, which is conducive to produce precipitation.