Abstract: In this paper, multisource high-resolution data are used to analyze the triggering mechanism of convection at the second stage of the most vigorous development of convection during the heavy rain in Ya’an, Sichuan, on August 4, 2021. The conclusions from this study are as follows. (1) The mesoscale disturbance system in the middle and upper troposphere provides favorable dynamic and thermodynamic conditions for the trigger and development of convection in the second stage of this heavy rain (2) The surface convergence line formed by the cold pool outflow can directly trigger convection, whereas that formed by mountain-to-plain flows and topographic circumfluence cannot directly trigger convection. Convection can be easily triggered only when there is a “secondary uplift mechanism” in the upper and middle layers of the boundary layer. This “secondary uplift mechanism” includes the coupling uplift mechanism of the ambient atmospheric convergence and uplift, forced uplift of high-altitude terrain, and upstream updraft in front of the moving residual weak convection system. (3) The positive vorticity zone formed on the northeast side of Mount Emei under the mechanical forcing of terrains is conducive to the strengthening and development of convection. Furthermore, the low-level jet generated by the strong convective system moves northward and meets the cold pool outflow south of Mount Longmen, triggering the “elevated convection.” This convection triggering mechanism is closely related to Mount Emei and nearby high-altitude mountains and may be a unique convection triggering mechanism in the eastern slope of the Qinghai–Tibet Plateau. (4) Under the action of different triggering mechanisms, the time difference and relative position of lift formation and convection initiation vary. Compared with the cold pool outflow and terrain uplift, the time difference is longer and the distance is farther under the influence of downhill wind and terrain flow..