Synoptic circulation pattern and mesoscale system associated with the extreme torrential rain occurring in Shandong Peninsula on July 22, 2020 are analyzed based on conventional observational data and high resolution numerical simulation by using the mesoscale model WRF. The simulation was verified to agree well with the precipitation process. The results show that the rainstorm process has the characteristics of mesoscale features spatially and temporally, represent in its high intensity of short-term rainfall, severe locality, etc. Precipitation occurs in the southwest airflow between the subtropical north elevation and the bottom of the low vortex. Strong vortex and low-level jets are important weather systems that affect this precipitation. The southwest jet stream is the main carrier of extreme water vapor during this heavy precipitation. Under the high-level weak divergent field, the main influencing factor of this rainstorm is the deep low vortex extending from the surface to the 500hPa high altitude. Its temporal and spatial evolution characteristics are consistent with the mesoscale cloud cluster changes shown by the FY-2E hourly TBB data. It is directly related to the occurrence of heavy rain. The interaction between the vortex, the low-level jet and the subtropical high strengthens the development of the low vortex. There are warm; wet and cold dry air flows in, from the north and south of the low vortex. The specific humidity gradient is roughly distributed from south to north, which is a typical flow field distribution of the vortex accompanied by a low-level jet. Convergence of low vortex and its interaction with the strong wind speed belt at the edge of the subtropical high together lead to the development and maintenance of strong vertical motion, together contributed to the persistence of extreme rainstorm.