Abstract: The WRFV2.2.1 model is used to investigate the mechanism for a severe snowstorm event occured in Liaoning Province during 3-5 March 2007. With the help of the digital Doppler radar data and the pressure data per 10 minutes from the surface automated mesonet stations, the structure and the environment features of the mesoscale gravity waves, as well as the source of the gravity waves are studied. The results show that the WRF simulated gravity waves developed in the upper layer of the troposphere between 350-250 hPa (9-11 km) with an period of 2-3 hours, the horizontal wavelength of 30-40 km, and propagating about 9 hours horizontally. The sea level pressure perturbation amplitude is about 2 hPa, with a wave period of 2-3 hours, and propagates from southwest to northeast. Along the wave propagating path, the observed 1 hour snowfalls show significant wave feature, with a period of 2 hours. When the WRF-simulated gravity waves is weakened, the Doppler radar reflectivity shows significant wave feature as well. The WRF-simulated mesoscale gravity waves were developed in an environment of strong shear instability at 300 hPa, under the exit region of an upper level jet streak propagating toward the ridge. The 300 hPa level where the gravity waves developed with Richardson number ＜0.25 is also featured of stronger warm inflow, weak wind, and strong wind shear. Unbalanced flow developed in the area of mesoscale gravity waves with Lagrange Rossby number ＞0.7.The magnitude of the horizontal divergence tendency is larger with the wind shear term contributed most. It is shown that the shear instability has strong effect on the development of the gravity waves.