Generation rescheduling is one of the main control approaches for improving transfer limit of a power system. In this paper， optimal condition of a generation pattern on maximum loading margin is derived. A new model for the maximum loading margin is presented according to this optimal condition， by which the optimization problem is transformed into a problem of solving a set of non-linear equations. The proposed model provides a certain foundation on the application of generation optimization on voltage stability. At last， the optimal correction direction for generation pattern is acquired accurately by building sensitivity matrix of normal vector respect to a generation pattern and selecting reference point carefully.