Using the wet Q-vector and combining with the intense data output of the modified MM4(MMM4) model simulation,a typical Meiyu front heavy rain process occurred during 5-6th July 1991 in the Changjiang-Huaihe Basin is diagnosed carefully in this paper. The results are as follows:the convergence field of wet Q-vector divergence tilts northwards with height; the divergence/convergence fields of wet Q-vector divergence show an alternate distribution in the vertical direction at the developing and decaying stages of Meiyu front heavy rain; based on the convergence field of wet Q-vector divergence,the intrinsic mechanism of Meiyu front heavy rain is discussed, and a new thinking for the forecast of Meiyu front heavy rain is given.