In this paper, using multi-source high-resolution data analyzes the triggering mechanism of convection in the second stage of the most vigorous development of convection during the heavy rain in Ya"an, Sichuan on August 4, 2021. The conclusions are as follows: (1) The mesoscale disturbance system in the middle and upper troposphere provides favorable dynamic and thermodynamic conditions for the triggering and development of convection in the second stage of the process. (2) The surface convergence line formed by the outflow of the cold pool can directly trigger convection, while the surface convergence line formed by the mountain-to-plain flows and the 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. The "secondary uplift mechanism" includes the coupling uplift mechanism of the ambient atmospheric convergence and uplift, the forced uplift of high altitude terrain, and the 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 terrain is conducive to the strengthening and development of convection. 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 are different, among which the time difference and relative position distance are longer under the action of the mountain-to-plain flows and terrain flow.