Abstract: Owing to a light snowfall on December 7, 2001, traffic jams and congestion were occurred in Beijing. A diagnostic study and numerical simulations using MM5 were conducted on this high impact weather. The result of diagnosis shows that the weak trough at 500 hPa which moving eastward slowly and the 850 hPa high which brought in suitable water vapor from the South China Sea and East China Sea resulted in the happening of the light snowfall in Beijing. The results of the simulations indicate that the case can be simulated successfully by the numerical model MM5. The successive development of the background filed, the trigger mechanism and the water source, as well as the snowfall in Beijing, were well reproduced. Based on the results of diagnosis and simulations, possible factors contributed to the traffic jams and congestion were also discussed. Thus study on city disaster meteorology, i. e. high impact weather and setting up a monitory and forecast system are inevitable, which can synthetically evaluate the impact of meteorology phenomena on city life in China.