The northwestern Pacific is the only region where tropical cyclones (TCs) are generated all year round. China is one of the countries which are badly affected by TCs. A air－sea coupled model has been developed, focusing on the source region of the northwestern Pacific western boundary current system. Here, typhoon Kaemi is taken as an example to study air－sea interaction. The results show that: The SST cooling induced by the air－sea interaction weakens the typhoon, and the model results are consistent with observation during early simulation. However, 42 h later, the simulated typhoon is weaker than the observation. The typhoon track is scarcely influenced by the air－sea interaction. The SST cooling extent is determined by the relationship between typhoon moving speed and intensity and the maximum cooling is more than 4℃. The SST cooling zone drops behind the region of maximum wind speed along the typhoon track. The pumping and mixing induced by winds around the typhoon center bring cold water up and get the SST cooling. Such cold wakes last more than 54 hours. The changing of SST determines heat fluxes transport. Once the SST is constant, the heat fluxes are determined by the low-lying wind. Vertical convection is closely related to the typhoon intensity. Before 42 h, the ascending convection is the reason of typhoon intensification. Hereafter, the downdraft results in typhoon weakening. The coupling adds asymmetry of typhoon vorticity, and prevents its trend of ascending much higher. Maybe this is the mechanism of thermal factor affecting the dynamical structure.