The mechanism analysis of a floodinducing heavy rainfall occurred in June 2005 Guangdong Province is diagnosed by analyzing the circulation situation, moisture, dynamical mechanism, vertical helicity, and so on based on the conventional observation data, radar echo, satellite image and NCEP grid data. The results show that the causes of the heavy rainfall include the farther south and weakerthannormal ridge position of the West Pacific subtropical high and the interaction between the southwest warm and wet airflow to the north of the subtropical high and the cold air in front of the westerly trough; the lowlevel jet stream provides enough moisture; the divergence in high level and the convergence in low level resulted in strong updraft; the vertical helicity is two orders of magnitude greater than normal with the long axis of the positive vertical helicity area having the same direction with the movement of the shear convergence line on 700 hPa. During the heavy flooding rainfall, there was an enclosed negative center of vertical helicity at the middle level and an enclosed positive center at the upper level, spreading obviously down to the lower level.