Severe downslope windstorm triggered by the interaction between the gap jet and the asymmetrical topography in the Tianshan Mountain canyon caused severe disasters of trains rollover in the Turpan Depression in Xinjiang on February 28, 2007. To understand the mechanism of downslope windstorm between the interaction of large-scale circulation background， mesoscale system and complex topography in this extreme windstorm, we use a WRF model and conduct numerical simulation and mesoscale diagnostic analysis. The results show that under the pressure gradient between north-south sides of the Tianshan Mountain, the air parcel climbed windward slope and flew into the Tianshan Mountain canyon and then formed gap jet due to effect of funnelling, and at the same time， the jet generated gravity waves forced by the asymmetric terrain of the Tianshan Mountain canyon, and produced lee waves in the leeward, which transmitted the energy of the gap jet to the ground， forming the severe downslope windstorm finally. In this process, the turbulence formed by the wave breaking and the critical layer absorbed the upper layer energy downward, strengthening the energy of the gap jet. Moreover, the atmospheric stability stratification exacerbated the sinking movement which sank energy to the surface.