Abstract: In order to explore the relationship between moist potential vorticity field characteristics and Beijing ‘7.21’ heavy rainstorm, using NCEP/NCAR reanalysis data, the author conducted a diagnostic analysis of moist potential vorticity for the ‘7.21’ torrential rain process in Beijing. The results showed that: the favorable configuration that the cold air in high MPV1 value center of the middle and high level troposphere superimposed on the negative MPV1 of the middle and low level troposphere, which was very beneficial for the potential instability energy storage and release over Beijing region, and the development of the heavy rainfall in Beijing. The strong precipitation was closely related to the negative center of MPV1 and intensive band of MPV1. The intense precipitation in Beijing correspond with the positive area of MPV2, and the direction of movement of its precipitation zone coincided with the direction of movement of the center of positive MPV2. While the negative MPV2 downward extension degree, might be closely related to the Beijing precipitation intensity. Negative MPV2 region was also consistent with frontal direction. The mechanism of the formation of atmospheric instability over heavy rain area was that the high and low level systems coupled configuration, the warm air vertical motion triggered the convective instability energy release, and the ascending motion had been strengthened. The atmospheric convective stability in middle and high level troposphere, while the conditional symmetric instability stratification in the high and low level troposphere. So that the airflow tilted up to upper level troposphere, and maintained the strong precipitation in Beijing.