It is difficult to forecast the returnflow weather which occurred over North C hina. In order to reveal the mechanism of returnflow weather, such as the character o f returnflow, a heavy snow of returnflow weather which occurred over North C hina was diagnosed and investigated by using NCEP data and the Pennsylvania State Univers ityNCAR FifthGeneration Mesoscale Model (MM5), version 3. The results show t hat the start and end time of precipitation was closely related to the wind direct ion, including the upper warm and humid air and lower cold air. The lower layer air from Northeast Plain through Bohai Sea was drier and colder than the upper air f rom southwest. The moisture of returnflow weather mainly came from southwest w ith southwestern flow.