The unbalanced power caused by AC/DC faults in large power grid may cause local power grid to exceed the stability limit, or the voltage of key buses to exceed the voltage limit. In serious cases, it may lead to cascading faults or large-scale blackout. In order to accurately simulate the power flow distribution when the power grid transits to a steady state after faults and determine the validity of control strategy that controls the unsafe power fault, a rapid power flow calculation method after fault considering the automatic control strategy is proposed in this paper. According to the influence of automatic devices such as safety control system, system protection device, primary frequency modulation of generator, automatic generation control (AGC) and automatic voltage control (AVC) on the active and reactive power injection of grid nodes after AC/DC fault at different time scales, the action timing and strategy of various automatic devices are simulated to form the system power flow from fault occurrence to steady-state process, so as to determine the overload equipment, the stable section of power flow exceeding the limit and the key bus of voltage exceeding the limit, which provides a basis for the online control decision of large power grid. The effectiveness and practicability of the method are verified by the case analysis of the actual online data of power grid.