Abstract: By using upper air observational data, surface observational data, MICAPS 1°×1° objective analysis data and Kunming Doppler Radar echo data, 4 heavy snow processes in Yunnan since 2000 are contrastively analyzed and diagnosed. The results show that in the 4 heavy snow processes in Yunnan, 2 processes aroused by the transversal trough pattern affect the western area of Ailao Mountain, the snow area is large with the strong intensity, the other 2 processes aroused by the north ridge and south trough pattern mainly affect the middle and the east Yunnan. 3 heavy snow processes accompany the southern trough. Warm and moist airflow in front of the south trough converges with the strong cold air, producing heavy snow. This is the major heavy snow pattern of Yunnan. But only one without south trough in 2005 is very special. Before and during the 4 heavy snow processes, the water vapour flux increases quickly. Most areas in Yunnan are in the big value band of the east—west water vapour flux, which indicates that plenty water vapour transports to Yunnan from the west whether the south trough exists or not. In 3 processes with active south trough, the water vapour comes from the southwest Bengal Bay and this is the main heavy snow pattern in Yunnan. In the heavy snow process without the south trough influencing, the water vapour comes to Yunnan from the west of Arabian sea along the India high pressure. The strong water vapour flux convergence belt and area can better indicate the heavy snow processes of Yunnan. The heavy snow always appears in the area of strong water vapour convergence and closes to the side of strong cold air in the east of the convergence center. Because the updraft and the downdraft in the south and the north coexist, the mesoscale vertical circulation near the front area may establish. When the cold air from the north moves to the south, the θ_se sharpness area also moves to the south, and then causes the water vapour brought by the south warm moist airflow to condense and the unstable energy to release, providing enough uplift condensation condition for the heavy snow. In the Doppler radar echo intensity field, the sheet cloud echo arouses the heavy snow, the intensity of sheet cloud echo is about 20 to 30 dBz. In the velocity field, the cold and warm advection's alternative change and the stronger southwest jet stream of the upper air in the middle and lower layer of troposphere are the major macro-scale characters of the heavy snow with the south trough in 2004. The partial east jet stream in the lower layer is the major macro-scale character of the heavy snow without the south trough in 2005. So the high and low level jet stream are the key to the heavy snow processes.