From data analysis and numerical validation, the influence factors of water level dropping in Nanchang reach of Ganjiang River were analyzed in the aspect of the water and sediment discharge from the upstream, water level dropping of the Poyang Lake in the downstream and the riverbed deepening in Nanchang reach. It shows that riverbed deepening caused by the large scale of artificial sand mining is the main influence factor for the water level dropping in Nanchang reach. Water level dropping of the Poyang Lake is another factor and the dropping amplitude varied from the amplitude of the water level in the downstream, the distance from the Poyang Lake and the river discharge. In addition, the water level drop in the downstream is also evidently affected by local high beach bed in the dry season. Self-scouring only accounts for 1. 0%~3. 5% of the riverbed deepening, which has limited impact on the water level dropping in Nanchang reach.