A sand-dust numerical model coupled with GRAPES (Global/Regional Assimilation and Prediction Enhanced System)－GRAPES_SDM is introduced. GRAPES_SDM includes such detail physical processes as dust emission, transport, dry deposition and clear sky process, which can simulate and forecast the initial and sand-dust concentration of sand and dust storm. Based on GRAPES_SDM, a numerical model forecasting system is established in northwest China, and is applied to Lanzhou Center Meteorological Observatory for operational prediction of sand and dust storm from April, 2005. Two severe sand and dust storms occurred in 17-19, April, 2005 and 28-29, May, 2005 in northwest China are choosed to simulate there initial, transport, dust emission and disappear. The results show no difference for sand-dust spetial distribution between model outputs and remote sensing monitoring imageres, which indicates that the model system has the capability of forecasting sand and dust storm in northwest China.