The invention discloses a transient stability constraint power system flow optimization method a based on a difference equation constraint transformation order-reduction technology. The method comprises the following steps: after the transient stability constraint is considered, a considerable amount of state variables are increased for the flow optimization problem of the power system, and the scale of the problems is greatly increased; if a plurality of contingencies are considered at the same time, the curse of dimensionality possibly appears, so that the optimization problem cannot be explained. The invention transforms the partial or all the difference equations into inequality equations according to the demands of the realistic accuracy, and selects the inequality equations relative to the precision of the difference method to restrain the upper and lower limits. The computation time can be obviously reduced by dealing with the flow optimization problem. The method can handle the flow optimization problem of the power system with multi-contingency transient stability constraint considered in the larger-scale power system.