The invention provides a two-layer optimization method for a security constrained unit commitment based on Benders decomposition. The two-layer optimization method comprises the steps of: (1) determining a unit commitment state main problem taking unit start-stop states as decision variables; (2) performing network security constraint checking at an arbitrary moment; (3) and solving the optimal economic dispatching subproblem taking unit output as a decision variable. The two-layer optimization method calculates the optimal unit commitment satisfying a network security constraint condition through adopting a provided two-layer optimization model based on Benders decomposition, and solves the optimal economic dispatching problem through iterative calculation. Since the unit commitment U0 and the unit output P0 are optimized separately, the two-layer optimization method increases the optimization space and capability, and the final solution is better than that of the traditional optimization algorithm.