This paper presents a new generation rescheduling approach for preventive control of power systems, which can optimally reallocate power generations for multiple unstable contingencies. The transient stability constraints used in the optimal rescheduling model are described by a heuristic stability performance index. A backward integration technique is proposed to evaluate gradient of the performance index. Using the gradient, the functional inequality constraints imposed by unstable contingencies are expressed by linear inequalities, and the rescheduling problem is solved iteratively. Effectiveness of the approach is demonstrated by numerical tests on a representation of North-China power system. Index Terms-Optimal control, power generation control, power system security, power system transient stability.