Voltage stability margin is an important index to evaluate the security and stability of power system. In view that voltage stability margin’s solution using normal nonlinear programming method does not consider the growth mode of generators’ active power outputs, the results of load margin are too ideal. A dynamic nonlinear programming method model considering the growth mode of generators’ active power outputs is proposed based on dynamic power flow. In the model, the unbalanced active power is distributed by the growth mode of generators’ active power outputs which is based on the actual situation. So the results of load margin calculated by the dynamic nonlinear programming method are more practical and not affected by the selection of slack node. Tests on IEEE-30 system and Liaoning power system show that results of voltage stability margin calculated by dynamic nonlinear programming method are more reasonable and practical than normal nonlinear programming method.