Abstract: Review was carried out in this paper to investigate the reasons for underestimated rainfall of the super typhoon Meranti (1614) in coastal Zhejiang province, and water vapor trajectory clustering was done as well by using the NOAA HYSPLIT4 model to investigate the quantitative contributions of water vapor to rainfalls from different channels. Results show that typhoon track deviations and rainfall underestimations by numerical models had direct impacts on the subjective forecasts of rainfall intensities and hitting areas. Moreover, numerical model forecast deviations, including weather patterns at mid-latitude, a typhoon reverse trough at coastal Zhejiang areas, and the eastward track offset of typhoon Malakas (1616), which led to the neglect of the interactions between Meranti and other weather systems in subject forecast and impacted the correct forecasts of rainfall dynamics and vapor conditions, should all be blamed for the rainfall underestimations. There were three water vapor trajectories for Meranti, from the northeast, southeast and south oceans (seas) respectively, with the latter two being the main sources. The southeast airflow was the most important for the rainfalls at the northern coast of Zhejiang, which took up more than 70% of the contributions to water vapors and rainfalls, implying the important role of typhoon Malakas for the rainfalls of typhoon Meranti. While at the southern coast of the province, the south airflow took almost as much role as the southeast airflow in water vapor transportation. Analysis implied that in routine services attentions should be paid to the interactions between typhoons and other weather systems as well as binary typhoon interactions.