Abstract: A typical MCS (mesoscale convective system) forming over the second-step terrain, moving eastward, merging with convection from the eastern edge of the Qinghai–Xizang Plateau (QXP), and influencing downstream convection systems was analyzed through numerical simulations. This case occurring from 29 June to 1 July 2016, caused heavy precipitation from the middle reaches of the Yangtze River to the Yangtze–Huai River basin. This simulation successfully reproduced the eastward movement of the MCS over the second-step terrain and its merger with the convection moving eastward from the QXP, thus affecting downstream convective systems. The results revealed the spatiotemporal patterns of precipitation across the middle and lower Yangtze River regions. The evolution of the MCS is classified into four stages: Formation, propagation, merging, and weakening. Initiated by a cyclonic disturbance in the northeastern part of the Southwest vortex, the MCS formed over the western mountainous areas over the second-step terrain. Upon exiting the second-step terrain, the MCS merged with eastward-propagating convection from the QXP over downstream areas. This merging promoted the strong development of upward motion, enhanced low-level convergence, and generated a mesoscale vortex in the lower troposphere. The integration between the mesoscale vortex and the merged convection promoted their coupling and triggered heavy rainfall in the eastern plains beyond the second-step terrain.