In order to solve the problem of contingency screening that power system faces, an improved contingency screening strategy is proposed. This paper uses the analysis method of dynamic security domain, and practical probabilistic insecurity index is a basis for contingency screening and ranking. Building models depend on fatal contingency or not, then it takes the loss that may be caused by the failure as the objective function to rank. This method analyses different circumstances under a variety of fault classification that the system faces and considers the overall probability distribution, reflects system security by weight control and economy index by objective function. Then it uses the method of off-line calculation and online application to get the results of contingency screening. Tests on the New England 10-generator 39-bus test system show that the results accord with the factual basis, and it has good robustness and strong guidance for the processing of actual hidden faults.