Under the condition of given correction direction， the model for optimal step size is developed first. In addition to the restriction imposed on the permissible correction angle between two sequent generation-directions， a fairly satisfactory step size can be determined by iteration to the constructed model only once， by which the computing effort can considerably be reduced while maintaining convergence precision. Then the generation direction（GD） is updated according to the known correction direction and step size. The collapse point is re-calculated under the new GD. The load margin and GD would gradually converge to the optimum by this alternative optimization method. Finally， the speed and efficiency of the proposed method is verified on multi-IEEE standard test systems， with the effect of generation dispatch optimization on improving the load margin summarized from the simulation results.