An improved method for transient stability-constrained optimal power flow is presented in this paper. The finite dimension transient constraints are transformed into one dimension inequality constraint in this method， so that it significantly reduces the computational burden and improves the calculation efficiency when multi-fault is resolved. A standard deviation criterion is proposed to measure the transient stability. The rationality of the substitute criterion and its robustness are analyzed. The proposed method has been validated in 10-machine， 39-bus test system.