The operation of a power system will inevitably be influenced by a variety of natural disasters， e.g. typhoon and forest fire. These disasters will dramatically change the operating environment of a power system and increase the occurrence probability of some specific faults. It is important to make use of the disaster's forecasting information by combining the current operating state of the network and some important components， to make security analysis and risk evaluation， so as to establish the forewarning mechanism， emergency response mechanism， rapid post-disaster restoration and reconstruction mechanism in case of a natural disaster. As the security degree is measured by estimating the influence of contingencies on the power system， the predictive contingency set has a direct influence on the security evaluation results and control strategy. The contingency selection and its ranking method according to particular natural disaster forecasting information are discussed. It is expected to help achieve on-line security forewarning and adjust the pre-decision of dispatching according to the development of the specific disaster.