A sanddust weather event in Northwest China was diagnosed and analyzed by means of the routine observation data and WRF model outputs. The results show that this event belongs to Mongolia Cold Trough, and the main cause of the sanddust event was the invasion of strong coldair mass on surface, in which the sanddust weather occurred at the rear of a ground cold front during the transit of the front. The development and movement of the upperlevel jet was indicative of sanddust weather. The vertical secondary circulation changed evidently at the same time, and ascending and descending air currents coexisted. The matchable divergence field enhanced the ascending and descending movement. The vorticity field was important because it could affect the Mongolia Cyclogenesis. The stable stratification and distinct lowlevel vertical wind shear were two features during the sanddust event. The WRF model performed well in predicting this event.