The limited transmission capacity of some lines occurs in power grid construction, with facing the problem of "insufficient grid inertia" caused by high proportion of power electronic equipment entering the power grid. In order to solve these problems, a novel motor-generator series transformer (MGST) power flow control method is proposed. The control method can not only realize active power flow control and reactive power compensation, but also provide inertia support for the power grid. Taking advantage of the condition that the synchronous motor unit itself has large inertia, its installation mode and the excitation control strategy of the motor are adjusted. Finally, the equipment is simulated and analyzed based on MATLAB/Simulink platform, and the results are compared with the simulation results of unified power flow controller (UPFC) device under the same situation. So, the adjustment effect of MGST device is analyzed. The simulation results show that the MGST device has the ability of controlling power flow and keeping voltage stable. However, due to the large inertia of motor rotor, the response speed is slightly lower than that of UPFC. As a novel power flow controller, the MGST device can achieve the expected adjustment effect at present.