Abstract: A local extremely heavy rainstorm occurred in Changtan Town, Wanzhou District, Chongqing, from 14:00 BT on 3 to 14:00 BT on 4 July, 2023 (Beijing time, the same below). The hourly heavy rainfall exceeding 40 mm for four consecutive hours in the early morning of 4 July triggered geological disasters and caused casualties. Using data from surface weather stations, FY-4A satellite, Doppler weather radar, CLDAS fusion data and ERA5 reanalysis data, this study analyzed the circulation background and environmental conditions that led to the local extremely heavy rainstorm. The characteristics and causes of mesoscale convective system (MCS) were discussed. The key factors and conceptual model influencing this process were summarized. The results are as follows. (1) During the eastward retreat of the subtropical high, the stably maintained low vortex shear line was formed by the warm and humid airflow around the subtropical high converging with the cold air guided by the high-altitude cold trough, which provided favorable dynamic and moisture convergence conditions for the occurrence and development of mesoscale and microscale convective systems. Meanwhile, the high-instability energy tongue provided the necessary thermodynamic conditions for heavy rainfall. (2) The MCS was formed by the combination of the small scale convective cloud clusters in the west of Wanzhou and the convective cloud clusters in the southwest of Chongqing, which caused the heavy rainfall in Changtan Town, Wanzhou District. The local heavy rainstorm was mainly due to the "train effect", which was generated by the eastward movement of the stable and sustained "southwest northeast" oriented band echoes in the central and western parts of Chongqing and the convective cells in the western part of Wanzhou District merged and strengthened. (3) The topography effect of Qiyao Mountain in the east of Wanzhou was favorable for the dynamic lifting of the frontal zone, which enhanced the vertical uplift motion and water vapor aggregation. Moreover, the superposition effect of mesoscale topographic convergence line formed by Fangdou Mountain topography further promoted the stability maintenance and enhancement of MCS, ultimately triggering the local extreme rainstorm in Changtan Town, Wanzhou District.