According to the occurrence and maintenance of torrential rain relating closely to moisture convergence increment in low layer of troposphere, factors affecting the evolution of divergence is analysed to reveal the dynamic mechanism of heavy rainfall maintenance. Starting from the primitive equations, we obtain a new type of divergence equation including the coupling forces between Q vector and vertical shear of wind. Analysis on this equation brings to light the main factors to influence the evolution of divergence. They are separately the barotropic non-equilibrium force and the baroclinic coupling forces between Q vector and vertical shear of wind. Especially, the evolution of divergence is only controlled by the above-mentioned two factors on the layer with the maximum convergence . In addition, the definite integral for new type of divergence equation is formed from the earth’s surface to the un-divergent level. Results show that the self-exciting process of divergence fields lead to the decrease of convergence in the low layer of troposphere. Oppositely, the barotropic non-equilibrium force and the baroclinic coupling forces between Q vector and vertical shear of wind are the main factors that promote the increment of convergence in the air column, which we should pay more attention to in studying the mechanism of heavy rainfall maintenance.