The paper discusses the structure of the disaster risk system. There are three main methods in risk assessment about natural disasters: the historical disaster data based method, the indicator system based method and the method based on scenario simulation. The paper deducts the register of historical disasters, according to the idea in international risk assessment programmes about natural disasters. The paper concludes some regional differentiation rules of flood vulnerability and discusses the reasons of these phenomena in order to find out the origin of disaster vulnerability from the human society. There are not enough historical disaster data, so an information diffusion based fuzzy method is introduced to optimize the historical data and proved to be a practical method for risk assessment of natural disasters. After the order of risk and vulnerability are compared, vulnerability is proved to be a pivotal tache to risk. It is also important to find out the rule of floods and reduce the exposure of human beings to decrease the risk.