Using high-resolution simulation produced by the WRF (weather research and forecasting) model, three dimensional precipitation equation, and definition of precipitation efficiency, an heavy rainfall event occurred in Mianning on 26 June 2020 is studied. The vertical dynamic structure, hydrometeor distribution and various physical processes of precipitation are compared and analyzed. The characteristics related to water substances ( water vapor, rain, snow, hail, graupel, ice and liquid cloud) in complex terrain areas are further revealed.The results indicated that the rainstorm process can be divided into two stages. In the first stage, from 1800 BJT (Beijing time) to 2200 BJT 26 June 2020, The sharp ascending motion in the strong precipitation area promotes the convergence of water vapor and liquid-phase hydrometeors from the weak precipitation area. The abundant liquid-phase hydrometeors collected in the strong precipitation area are converted into heavy rainfall and ice-phase hydrometeors to supply cloud growth. At this time, The strong convective echo with the reflectivity of ＞35 dBZ reached the highest level. In the second stage, from 2300 BJT 26 to 0100 BJT 27 June 2020, the local ascending motion in the strong precipitation area is weakened, which leads to the weakening of the convergence of liquid-phase hydrometeors , but the convergence of water vapor is still strong and the location of maximum ascending motion is descended, which is helpful for the condensation of water vapor and the collision of clouds and raindrops. There are two maximum centers of ascending motion in the vertical direction. The echo intensity shows profile of strong-weak-strong. The ice particles in the upper atmosphere may be as seeds for the lower cloud, which is helpful for the conversion of liquid-phase hydrometeors into heavy rainfall. The precipitation efficiency varies significantly among different rainfall intensity ranges, from 5 % to 70 %. The greater the precipitation efficiency, the stronger the surface precipitation.