Abstract: Diagnostic analysis was applied for a continuous rainstorm event in Gannan Gansu Province, July 2-5, 2011. The results demonstrated that: this rainfall event was related with the shear on 500 hPa and location of strong convergence zone on 700 hPa. Low-level strong convergence provided a favorable dynamic condition for the occurrence and evolvement of heavy rainstorm. Through the analysis of the physical field, it was found that during this continuous rainstorm, there existed an obvious moisture layer and convergence of water vapor in low-middle layer, thereby providing adequate moisture condition for precipitation. Rising in the southwest airflow movement around the periphery of subtropical high, the water vapor and energy were transported eastward to precipitation area, which was the main cause of the continuous rainstorm event.