Abstract: Variable frequency transformer (VFT) is a new type of flexible AC transmission system equipment to achieve the interconnections of asynchronous grids, whose transmittable power capacity is an important performance indicator. In view of the problem that the traditional control strategy does not consider the rotor winding’s overloading caused by the increase of slip rate, a study is conducted on the maximum stator active power under different slip rates and stator reactive power, and the operation characteristics of a VFT system based on series converter are investigated. The detailed working conditions are analyzed for different slip rates. The mathematical relationship between the optimal stator reactive power reference and the slip rate, and the rated apparent power and the actual absorbed reactive power is derived, and the maximum power transfer (MPT) control strategy for VFT is proposed. A hardware-in-loop (HIL) experimental platform is established to verify the effectiveness of the proposed MPT control strategy. The experimental results show that the proposed MPT control strategy can effectively increase the transferable power capacity of VFT and realize the maximum power transfer of VFT.