Contingency is inevitable in a power system.Nevertheless，it means a great deal to operators to analyze the causes，the distribution of contingencies and characteristics of serious failures when studying the mechanism of contingency occurrence，finding out the influence of contingencies and then improving the pre-control capability of operation risk.The statistics and distribution characteristics of contingencies of500kV transmission lines in North China power grid are analyzed from several perspectives，as is the correlation between different kinds of contingencies and all kinds of factors.The characteristics of four serious failures during2012are analyzed at length，and the inspiration of these serious failures is well concluded.In the end，the risk prevention and control method is proposed from the perspective of power system operation.The corresponding key technologies are discussed，and suggestions for actual operation are made.