In this article, the authors used the Weather Research and Forecast model to investigate the sensitivity of tropical cyclone Bilis’ total precipitation to ambient water vapor content. The tropical cyclone precipitation decreased dramatically with the reduction of the ambient water vapor content in the atmosphere. The decrease of precipitation can be explained by two mechanisms. First, as the major source of precipitation, moisture convergence decreases accordingly with the moisture content. Second, the tropical cyclone (TC) size reduction due to the moisture decrease influences the precipitation area. Both mechanisms were observed in this study.