The present disclosure provides a method and a device for solving an optimal power flow in a power supply system. A second convex model of the optimal power flow is established. A relaxation variant sum e according to the second convex model is determined. It is judged whether the relaxation variant sum e is greater than a preset threshold. If the relaxation variant sum e is greater than the preset threshold, the second convex model of the optimal power flow is established. If the relaxation variant sum e is not greater than the preset threshold, the solution of the second convex optimal model is determined as a feasible solution of the optimal model of the optimal power flow.