The invention discloses a power flow optimization method for an electric power system based on subarea division and class extension variable relaxation. The method comprises the following steps of decomposing the electric power system to obtain a plurality of subsystems which are mutually independent; coordinating the subsystems to obtain a decomposition coordination model of the electric power system based on subarea division; adopting an auxiliary problem principle to carry out distributed parallel optimal power flow computation on the decomposition coordination model of the electric power system; and adopting a class extension variable interior point method to solve the subsystems respectively to obtain an optimal solution of power flow of the electric power system. The method conforms to the characteristics of scattered data and structural distribution of the large-scale interconnected electric power systems; the dimensionality of the subsystems is effectively reduced; the optimal computation of each subarea only needs data and boundary node data of the area and does not need a detailed mathematic model outside the area, so that the bottleneck of data transmission is avoided and the optimization speed is accelerated; through the class extension variable interior point method adopted by a subsystem optimization problem, a solution space of the optimization problem is expanded and the convergence is improved.