Based on quasi-geostrophic Q-vector theory,a case of continuous heavy rain has been analyzed. Q-vector, vorticity and divergence of Q, geostrophic frontogenesis function have been computed in 13 times of the heavy rain case. Results show that the internal mechanics of the heavyrain is a contrary movement of warm and cold air mass with clockwise and spiral lift movement.Further more, the forecast method of heavy rain area with Q-vector field in operational forecast practice is indicated.