Abstract: An extreme torrential rain event occurred in the Jianshi Canyon in the mountainous area of Southwest Hubei on 26 July 2020, which was mainly caused by quasi-stationary mesoscale convection system (MCS). Using conventional observations, the data from regional automatic weather stations, radar data and ERA5 reanalysis data, we have performed analysis on the atmospheric circulation and the evolution and formation mechanism of quasi-static MCS in this event. Results are as follows. (1) Affected by the continental high and the Mongolian cold vortex moving eastward and southward and the stable subtropical high, the MCS forms in the north of the subtropical high and near the low-level convergence line. The mid-and low-level convergence line and the southerly jet in front of the low vortex provide favorable environmental conditions for the development and maintenance of the MCS. (2) The MCS in the Jianshi Canyon shows the characteristics of quasi-stationary with backward propagation, whose direction is consistent with the canyon trend, and it experienced two stages: quasi-stationary backward propagation and quasi-stationary merging. (3) Under the complex terrain of Jianshi Canyon and Enshi "bell mouth", the interaction between cold outflow and continuously strengthened and sinking low-level southerly jet has a significant effect on the formation and development of the MCS. The low-level jet is forced to rise on the windward slope for sustained weak cold outflow, forming a stable lifting condition in front of the mountain, which is the main reason for the maintenance of quasi-stationary backward propagation. The blocking effect of the low-level jet and the terrain in the south of Jianshi on the cold pool, and the merging of convective cells moving northward, are the reasons for the formation of quasi-stationary merging of the MCS. (4) The complex terrain plays an important role in the evolution of the MCS. The canyon wind effect and the topographic lifting effect of windward slope result in the enhanced development of the MCS. The cold outflow guided by the slope terrain of Jianshi Canyon quickly descends the mountain to form the convergence zone with the southerly lifted by the windward slope at front of the mountain, which speeds up the backward propagation of the MCS. The γ-mesoscale topography in the south of Jianshi Canyon blocks the downhill cold outflow and makes it move slowly southward, resulting in the propagation speed of the MCS to slow down and show a quasi-stationary state.