Abstract: The research status of drought severity assessment was reviewed firstly. It was noticed that the beginning of drought is the scarcity of precipitation, but there is a slow and gradual evolution course to cause final disaster loss. The key factors are not only precipitation and temperature, but also the landform, soil, crop and others. By using distributed hydrological modeling, a drought model was introduced to simulate the drought developing processes. Furthermore, according to the simulation results of the drought model, the method and index system of comprehensive assessment were proposed. The assessment method was demonstrated in the upper Yangtze River basin to review the evolution course of drought events of 2006.