The synoptic circulation pattern and mesoscale systems associated with the extreme torrential rain occurring in the Shandong Peninsula on 22 July 2020, are analyzed with conventional observational data and a high-resolution numerical simulation using the mesoscale model WRF. The simulation agreed well with the precipitation process. The results show that the rainstorm process is characterized by mesoscale features spatially and temporally, represented 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 a low vortex. Strong vortices 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 a high-level weak divergent field, the main influence of this rainstorm is the deep low vortex extending from the surface to the 500-hPa high altitude. Its temporal and spatial evolution characteristics are consistent with the mesoscale cloud cluster changes shown by the FY-2E hourly TBB data. This consistency is directly related to the occurrence of heavy rain. The interaction between the vortex, low-level jet, and subtropical high strengthens the development of the low vortex. There are warm, wet airflows from the north and cold, dry airflows from the south of the low vortex. The specific humidity gradient is roughly distributed from south to north, which is a typical flow field distribution of a vortex accompanied by a low-level jet. The convergence of the low vortex and its interaction with the strong wind speed belt at the edge of the subtropical high lead to the development and maintenance of strong vertical motion, thereby contributing to the persistence of extreme rainstorms.