A heavy rainfall in Hunan Province in June 2005 is diagnosed based on NCEP/NCAR reanalysis data and observational data. The results show that the most intensive moisture convergence, the strongest ascending movement and the most instable convection stratification all occur in the rainfall center. The heavy rain region locates at the rising branch of a secondary circulation forced by the convergence of the ageostrophic wet Q-vector. Descending flows prevail at the south and north sides of this convergence center and produce a compensation effect, which is favorable for maintaining the heavy rainfall. Convergence of the ageostrophic wet Q-vector has a forecast significance for the coming 6h rainfall area through producing a secondary circulation and reinforcing the storm. The rainfall occurrs at the warm and wet side of the negative value center of the moist potential vorticity (MPV) at 700 hPa and relative MPV at 850hPa. The warm and wet advection and the strong ascending movement at lower level lead to the compensation of the moist air and the upward transport heat, and strengthen the high-level divergence and low-level convergence, which ultimately reinforce the storm.