Abstract: Using the conventional ground, upper-air observation data and ERA5 reanalysis data, the environmental field and unstable conditions of a heavy rainstorm process in Guangxi from 19 to 20 July 2020 were analyzed. The results show that: (1) The first period of heavy rainfall was the rainstorm in the warm area of southwest jet stream, and the rainstorm points in northern Guangxi were scattered, which was sudden and local. The second period of heavy rainfall was frontal rainfall, the rainstorm area in central Guangxi was concentrated, the heavy rainfall duration was long, and the rainfall intensity was strong. Weak cold air moved southward in the early stage, and the heavy rainfall was located near the surface front. In the later stage, the cold and warm air confronted each other, the heavy rainfall was behind the front. (2) During the first heavy rainfall period, the convective instability stratified in the lower layer of northern Guangxi. The warm and wet transport of southwest jet and the invasion of dry and cold air from the middle layer promoted the convective instability to be gradually enhanced and maintained. (3) In the early stage of the second heavy rainfall period, the convective instability stratification was obvious in the lower layers of central Guangxi, and the cold air invaded, elevating warm and wet air, triggering the release of high convective effective energy. In the later stage, the convective instability in the lower layers weakened, the boundary layer southerly jet and precipitation enhanced the vertical wind shear and baroclinicity, and the symmetrical instability increased significantly, which was the dominant factor for the formation of rain belt and the persistence of heavy rainfall behind the front.