Abstract: During the heavy rain period, the large-scale vertical velocity is computed using the compaxatively complete co-equation. It is shown that the item of latent heat is the most important in various vertical velocity producing processes. The three-dimensional motion pattern through the central part of no. 3 typhoon, 1975, and the heavy rain area are shown. Some of the synoptical analysis conclusions for heavy rain, such as weak cold air and east air flow as well as tropical vortices upstream, are affirmed by the computed results. The variation of the structure of typhoon during the period of changes of direction of motion is shown. Finally, a scheme for prediction of of the heavy rain is proposed.