A severe snow storm on 3th December 2005 in the Shandong Peninsula is simulated with a mesoscale model MM5V3.6,and detailed diagnostic analyses are performed upon the mesoscale gravity-wave(MGW) event.The nonlinear balance equation(NBE) is used to diagnose the flow imbalance,and analysis indicates that the important role of the geostrophic adjustment and topographic forcing in the gravity-wave event in the snow storm.The MGW's wavelength is about 100km,and the propagation speed is about 20m/s.The Taihang Mountains and the flow imbalance are the key factors to initiate the MGW,on the other hand,the thermal forcing of the ocean in winter is also important in the genesis of the local snow storm.The NBE diagnostic shows clear evidence of the MGW initiation region and the propagation direction.The analysis on wave structure and maintaining mechanism suggests that the MGW in this snow storm satisfies the necessary conditions for wave ducting and Wave-CISK.This complex sequence of nonlinear interactions produced a long-lived,large-amplitude gravity-wave that created hazardous winter weather in Shandong Peninsula.