On the basis of direct current（DC） power flow model， the static security-constrained unit commitment problem is decomposed into two sub-problems，（1） the unit commitment problem without security constraints， and（2） the optimal power flow problem with static security constraints. Virtual variables are introduced into sub-problem（2） to reflect the restriction and influence of the unit commitment to transmission elements. The complementary constraints correlating the two sub-problems can be set up by the virtual variables and generation shift factors. This approach models the influence mechanism of sub- problem（1） varies with sub-problem（2）. It also provides an effective means to modify the optimization direction. Therefore， an algorithm for solving the two sub-problems alternatively is proposed. The proposed algorithm is sufficiently compatible with the present methods and conforms to the reality of the power systems， as well as has good flexibility to deal with the security restraint phenomenon of the unit commitment and evaluate the results of unit commitment problems.