The circulation patterns and radar echo feature of the heavy rain process in Liaoning Province, 5-6 August 2003, are analyzed. The simulation with PSU/NCAR MM5 model and a diagnostic analysis are made to investigate the development causes of the major influence synoptic system (North China cyclone). It shows that the cold air moved southward from Siberia, the stable and strengthened subtropical high and low level warm-moist air flow are the major causes of the event. The dry intrusion plays an important role in the rapid development of the North China cyclone.