Abstract: In this paper, the similarities and differences between flash heavy rain and heavy rainfall are discussed from the aspects of physical mechanisms and prediction techniques. The main results are as follow. (1) Abundant precipitable water vapor is a necessary condition to form heavy rainfall or extremely flash heavy rain. For non-convective large-scale precipitation or stratiform cloud precipitation events, the intensity of net water vapor advection or water vapor flux convergence in the lower troposphere is the core factor to judge the precipitation intensity, which is also the key factor in the analysis and forecast of large-scale rainstorm. For mesoscale stratiform cloud precipitation events, the vertical transport volume of water vapor and the net advection of water vapor in air columns are equally important. For the convective events, if the evaporation is not considered, the intensity of instantaneous precipitation is mainly determined by the vertical decline rate of water vapor (not the whole atmospheric precipitable water) and the Convective Available Potential Energy (CAPE) in the low-level rather than CAPE of the whole atmosphere. (2) The precipitation intensity is closely related to the effective condensation rate that forms effective precipitation at the ground, and which is directly related to the shape structure of cloud. From another point of view, the shape characteristics of convective clouds are determined by the atmospheric stratification and the vertical shear of ambient wind. (3) For the large-scale stable precipitation caused by synoptic scale systems, the duration of precipitation depends on the moving speed of the weather system, or more precisely, that of water vapor convergence over the precipitation area. For the convective precipitation events, the duration of precipitation depends on the scale, moving speed and propagation characteristics of the convective systems.