Abstract: Based on ECMWF daily reanalysis data, the mechanism of the heavy rain event occurred in Guangdong Province on 9 December 2015 was revealed by conducting circulation and diagnostic analysis. The results show that the southward moving dry cold air from eastern Europe has played an important role in exciting the heavy rain; the dry cold air flows around the Tibetan Plateau when flowing through; southern branch of the airflow around the Tibetan Plateau is entrained under the action of the anticyclonic circulation in the Bay of Bengal and the cold air reaches southern China via the Southern branch trough, which forms the main channel of dry cold air. Moist warm air comes from the Western Pacific before the heavy rain, and the easterly flow turns into southerly flow on Dec.9 2015, thus South China Sea becomes the main source of water vapor of the heavy rainfall. The convergence of cold and warm air masses contributes to the steep slope of potential pseudo-equivalent temperature. There is strong upward movement along the slope. But there is no deep potential instability in the ascending motion area. The southward motion of the cold air triggers the frontogenesis which centers in 700hPa. The terms of horizontal deformation and horizontal convergence have positive contributions to the frontogenesis. Frontogenesis stimulates positive secondary circulation centering at 700hPa, which contributes to the uplifting of the airflow, so the frontogenesis with the secondary circulation is the important mechanism of the upward movement in this heavy rainstorm region.