Using WRF model and three-dimensional precipitation diagnostic equation, the high-resolution simulation and diagnosis analysis of the physical process of heavy precipitation in the main precipitation period of the heavy rain process in Beijing happened on July 20 has been carried out. The results show that before the peak of precipitation, the strong water vapor convergence supports the strong precipitation, while humidifying the atmosphere. In the later stage, the water vapor convergence is significantly weakened, and the precipitation causes the obvious reduction of water vapor content in the local atmosphere. Before the peak time of precipitation, water vapor convergence, condensation and liquid-phase condensate convergence jointly contribute to the rapid development of heavy precipitation cloud system. In the later stage, the weakness of the dynamic convergence effect and the continuous consumption and divergence of water condensate lead to the significant decrease of water condensate content and the gradual disintegration of precipitation system. During the main precipitation period, the intensity and range of vertical upward motion gradually increased, and reached the maximum at the peak of precipitation, and then weakened and contracted. The peak height of the ascending motion is located on the zero level at the initial stage, and then decreases to the lower part of the zero level, accompanied by a "weak-strong-weak" precipitation intensity change. Under the control of ascending motion, the change range of water condensate is obvious, but the change range of different water condensate is different. Graupel particles and raindrops increase most significantly, and the contents reach the maximum at the peak of precipitation, and then decrease. The variation range of other water condensates is weaker than the above two due to the process of microphysical transformation and dynamic divergence. This paper also points out that the possible influence of different microphysical parameterization schemes on the physical process of heavy rain happened on July 20 and the differences of physical processes of precipitation with different intensities are worthy of further study.