Based on ERA-5 0.25°×0.25°reanalysis data, this paper calculated wet potential vorticity and frontier-generating function of the extreme rainstorm during 6 to 7 May, 2020 occurred on the north slope of the central Kunlun Mountains, the evolution characteristics of atmospheric instability in rainstorm process was analyzed, and the role of frontal system in convective triggering was clarified. The results showed: (1) During the heavy rainstorm period, the rainstorm area was controlled by the divergence superposition area caused by two stream jets at 200 hPa, the Central Asian vortex and the slowly-moving shear line in the north of the Plateau at 500 hPa. Additionally, the topographic convergence lifting at 700 hPa, and the confrontation of the cold air parcel in front of the surface high and the warm air parcel from Tarim basin, which provides favorable dynamic and thermal conditions for the development of the cloud cluster in the shallow mountain area on the northern slope of the Middle Kunlun Mountains. (2) The rainstorm process was divided into two stages. In the first stage(EP1), there was convective instability layer at the lower troposphere over Yutian to Qiemo area. In the second stage(EP2-1), there was a short-time heavy rainfall occurred in Cele station. The convective instability in the lower troposphere gradually translated to the symmetric instability, and the variation of Mpv2 at the lower troposphere was caused by the wet atmospheric baroclinic and vertical shear of the lower horizontal wind. Affected by the early short-time heavy rainfall and the release of latent heat of condensation, the lower layer atmosphere became warmer and moistened over Cele to Luopu area, and the lower layer atmospheric stratification stability turned into convective instability in the second stage(EP2-2). (3) The uplifting of terrain convergence and the frontogenesis at lower-upper troposphere in the northern plateau were the main reasons for the formation of mesoscale cloud clusters in the first stage. The cold front in the lower troposphere triggered the release of convective unstable energy, and the clouds near Qiemo developed rapidly. At the same time, the airflow which piled up in front of the hill climbed to the vicinity of 500hPa, the cold front frontogenesised and confronted the warm front on the north side of the plateau for a short time to form a cold occluded front. The vertical movement near the front enhanced the rapid development of convective clouds, and gradually merged with the convective clouds near Qiemo, resulting in the first stage precipitation (EP1) from Tian to Qiemo. During the second short-time heavy rainfall stage over Cele area(EP2-1), The cold front of 700 hPa in the lower troposphere frontogenesised, and the warm and wet inflow air in the direction of cloud movement met with the cold air mass of evaporation under the cloud and inflow behind the cloud. The warm air mass climbed further along the bottom cold pool, which made the cloud rapidly develop to mature stage, resulting in a short-time heavy rainfall over Cele area. As the Central Asian low vortex gradually entered the Northern Slope of the Middle Kunlun Mountains, the cold front frontogenesised in the lower troposphere and middle troposphere further strengthened the development of the upward movement, which was an important reason for the continuous precipitation from Cele to Luopu area in the second stage(EP2-2).