Abstract: A torrential rainfall process causing heavy floods occurred in Sichuan Basin from August 14 to 18, 2020. The circulation situation and causes of the process were analyzed by using radiosonde data, regional-station data and radar data, and focused on the role of the terrain in the torrential rain process in western Sichuan Basin. Results showed that: (1) The continuous heavy rainstorm process had the characteritics of continuous rainstorm weather scale feature with stable circulation situation of low value system. (2) In the continuous rainstorm process, the short-term heavy rainfall had a good relationship with the terrain, and the short-term heavy rainfall was distributed along the windward slope or bell mouth of the Longmen Mountains. (3) The terrain had great influence on triggering the heavy rainfall. Both the terrain convergence in the piedmont and the thermal difference of the underlying surface in the terrain transition zone could trigger precipitation. (4) The terrain had a significant enhancing effect on heavy rainfall. The warm and humid airflow accumulated on the eastern side of the mountain range, increasing the instability of stratification. At the same time, the easterly winds in the low level layers climbed along the terrain, promoting the development of upward motion and enhancing vertical effects through dynamic upliftth. (5) There was a feedback enhancement mechanism during the triggering and maintenance stages of heavy rainfall. When the slope direction of the terrain converged with the environmental wind direction, the warm and humid airflow climbed along the windward slope, generating upward motion, strengthening convection and increasing the rainfall. (6) The evolution of easterly airflow at 700 hPa and 850 hPa in the terrain of the Western Basin should be specially focused on.