With NCEP reanalysis data、 WRF model simulated high resolution output data、 automatic station precipitation data and radar data、 radar data, a diagnosing analysis and numerical simulation is made of a heavy rain in Chongqing from 14 to 16 August 2010. The result indicate that the heavy rain processes which divided two stages, has two precipitation system: the first stage of the main precipitation system is the low level shear line; The second stage of precipitation system are the low level shear line and 850 hpa of southwest vortex, and the second stage of the rainfall is larger and has longer duration. The vertical wind profile of time evolution shows that the heavy rain in the process of development, strong low-level southwest wind speed downlinking and the existence of the low-level shear convergence has a very important role for enhancement of the mesoscale convective system. The numerical simulation analysis shows that in the first phase of the heavy rain process, weak cold advection influence the rain area, under the impact of low-level shear line, cause air mass convergence in horizontal direction and trigger the mesoscale convective system development; in the second stage, the high-level strong cold air poured in precipitation area, the vertical ascending motion near the low vortex coupling interaction the upward movemen near 700 hpa shear lineafter the 850 hpa vortex eastward to Chongqing northeast area,which forces the low-level warm moist air to lifting and triggers the mesoscale convective system again.