Abstract: The extreme heavy rainfall weather under the long-term influence of MCS can easily cause serious disasters such as flash floods in complex terrain areas. Detailed study of its characteristics and causes can help better understand and defend against similar disaster weather. In this paper, we use multi-source meteorological observation data to study the MCS evolution characteristics and mechanism of two rainstorm processes on 10-11 August 2020 (process 1) and 4-5 August 2021 (process 2) in the complex terrain area of the eastern slope of the Qinghai Tibet Plateau. The results are as follows. (1) The two processes are warm rainstorm weather processes under the influence of weak weather scale system, and the strong precipitation areas are located in the Pingba River valley to the west of Luochun Mountains. The terrain has a very obvious impact on the strong precipitation areas. Compared with process 2, process 1 has larger precipitation intensity, wider range and more concentrated distribution of precipitation areas. (2) The MCS that caused the two rainstorm both originated in the high altitude windward slope terrain in the west of Ya'an, and maintained development in the west side of Luochun Mountains, but the MCS of process 1 was larger in scale, intensity and duration. (3) The heavy precipitation in process 1 is mainly caused by a larger scale convective (mainly cumulus clouds) MCS with stable and less movement, while process 2 is caused by two successively developing mixed (cumulus layer cloud mixing) MCS passing through the same area. (4) The maintenance mechanism of MCS during the two processes is different. The main mechanism of Process 1 is the continuous formation of deep convective cells under the forced uplift of the Luochun Mountain terrain, which merge with the original MCS1_A and maintain it on the western side of Luochun Mountain for a long time, forming a quasi stationary backward established MCS echo zone; At the same time, the blocking effect of the terrain on the outflow of the cold pool and the bypass effect on the warm and humid air flow also promote the stability and less movement of MCS1_A to a certain extent. The main mechanism for maintaining MCS in process 2 is the feedback effect of high-altitude windward slope terrain.