Using a WRF model and a three-dimensional precipitation diagnostic equation, a high-resolution simulation and diagnosis analysis of the physical process of the heavy precipitation during the main precipitation period of the heavy rains in Beijing was carried out on July 20 2016. Results show that before the peak of precipitation, strong water vapor convergence supports strong precipitation while humidifying the atmosphere. In the later stage, the water vapor convergence is significantly weakened, and the precipitation causes an obvious reduction in the water vapor content in the local atmosphere. Before the peak time of the precipitation, the water vapor convergence, condensation, and liquid-phase condensate convergence jointly contribute to the rapid development of the heavy precipitation cloud system. In the later stage, the weak dynamic convergence effect and the continuous consumption and divergence of the water condensate lead to the significant decrease of the water condensate content, thus resulting in the gradual disintegration of the precipitation system. During the main precipitation period, the intensity and range of the vertical upward motion gradually increased and reached the maximum peak of precipitation, after which it weakened and contracted. The peak height of the ascending motion is located at the zero level in 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 the water condensate is obvious, but the change range of a different water condensate is different. Graupel particles and raindrops increase most significantly; the contents reach the maximum at the peak of precipitation 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 differences of physical processes of precipitation with different intensities are worthy of further study.