This paper diagnostically analyzes the 721 Beijing rainstorm weather process caused by eastward shift of Hetao cyclone. The results show that: Hetao cyclone is the main impact system on the rainstorm in Beijing on 21 July 2012, and the forcing effect of upper divergence zone and the trend of lower troposphere frontal zone “guide” the Hetao cyclone to change the conventional path to the north, forcing its eastward shift and directly affecting the region of Hebei and Beijing. In the process of Hetao cyclone’s moving eastward along the frontal zone, the warm heart barotropic cyclones change to be baroclinic cyclones. Meanwhile, the potential energy of frontal zone is changed into kinetic energy to promote the rapid development of the Hetao cyclone, forming a positive feedback effect, and making the intensity of precipitation continuously enhance from west to east. The effects of the Hetao cyclone in the Beijing rainstorm are dynamic lifting, enhancing water vapor transport and triggering unstable energy. In the afternoon, warm front of the Hetao cyclone triggers the release of potential instability energy, generating strong convective system, forming mesoscale convective complexes (MCC) in the Beijing area, and finally, intensifying the precipitation further more. There are the main causes of the severe torrential rains in Beijing on 21 July. In addition, the topographic lifting in the southeast airflow strengthens the rainstorm as well. Therefore, this severe rainstorm is caused by a variety of superimposed favorable factors.