Abstract: Two representative typhoon processes are simulated using a Mesoscale Coupled air-sea Model (MCM). The quantificational effects of air-sea interaction on heavy rain and physical mechanism are analyzed and investigated in details. MCM is a two-way interactive mesoscale coupled model that linked two component models (i .e., LASG-REM or MM5 and ECOM-si) together via Unix's process communication te chnique and coupled object modules. The lower boundary of the atmospheric model is forced by SST which is produced by ocean model and the upper boundary of ocea nic model is forced by sensible heat flux, latent heat flux, net long-wave radiation and net short-wave radiation flux which are predicted from atmospheric model. The coupling is active both from 23°N to 41°N and from 116°E to131°E, and information exchange is in two-way over a time step of ocean model. Experime nts indicate that typhoon track, intensity, rainfall time, distribution and rain fall intensity are better simulated by MCM. Due to air-sea interaction involved in MCM, the intensity errors are decreased by 9.9 hPa at 48h and 3.5 hPa at 72 h, accumulative convective precipitation decreased by 40-100 mm for 24 h and the distribution of non-convective precipitation improved significantly. A potenti al mechanism that air-sea interaction impacts on typhoon heavy rain is a negati ve feedback regulated by SST reduction: Wind stress induces vertical mixing, leads to SST decrease, results in heat flux decrease and lower level cooling of atm osphere, weakens ascent motion, inhibits convective activity and rainfall via cloud physics, weakens typhoon intensity and maximum surface wind, feedbacks to ocean and then slower SST decrease, finally forms a negative feedback process.