Abstract: Thermal Convective Precipitation (TCP) usually occurs when the West Pacific Subtropical High controls China Mainland in summer. Under a back ground of prevailing subsidence, how can the convection start and where does the water vapor come from? The two issues are studied in this paper by selecting a typical TCP event taking place on 2 August 2003. Firstly, the characteristics of the TCP are illustrated by TRMM (Tropical Rainfall Measuring Mission) observations, automatic weather station observations and NCEP (National Center for Environmental Prediction) data. Then the surface evaporation, water vapor advection, and the divergence of the water vapor fluxes are calculated by the regionally-averaged water vapor budget equation, and the water vapor sources are analyzed. Furthermore, using the regional eta-coordinate numerical model AREM, the contributions of sensible heat and latent heat fluxes are investigated by 4 different sensitivity experiments. The results show that in the regions controlled by the Subtropical High, the surface temperature rises rapidly after sunrise. When receiving enough sensible heat, the air goes up and leads to the convergence in the lower atmosphere. Then the water vapor assembled from the surroundings and ground surface is transported to the upper levels, and the favorable environment for TCP forms. The simulation data diagnoses indicate that about half precipitable water comes from the convergence of water vapor horizontal flux, the other half comes from the surface evaporation, while little is from the advection. The further model sensitivity experiments prove that both sensible and latent heating are essential for the onset of TCP. The sensible heat flux triggers thermodynamic ascending motion, the latent heat flux provides water vapor, but the contribution to the TCP from the later is a little smaller than that from the former.