The current disaster assessment indicators are not comprehensive and systematic enough to evaluate the impacts of social, economic and environmental aspects, and the determination of index weight and grade assessment standards is subjective. As a result, a comprehensive evaluation index system of natural disaster grade assessment is concluded covering social, economic and environmental aspects. A cloud-based weight determination method and a grade assessment model of disaster situation are proposed and have been applied to six typical rainstorm and flood events. The results of analysis and evaluation show that the weight determination based on the cloud model can integrate all the assessing information from experts objectively. The assessment method can achieve evidence-based standard of disaster grades and determine the grade level objectively and reasonably.