Abstract: Connecting two asynchronous power networks by use of high voltage DC method has problems of harmonic interference, commutation failure, difficult implement of smooth power regulation and so on at present. The paper introduced basic structure and principle of variable frequency transformer, researched mathematic model and control model of the variable frequency transformer, and gave mathematic formula of transmission power of the variable frequency transformer and driving torque generated by DC driving motor. Then it built simulation model of the variable frequency transformer by use of PSCAD software and did simulation research. The simulation result showed that the variable frequency transformer can connect two asynchronous power networks and realize smooth bidirectional power transmission with quick response. Meanwhile, controlling speed of DC driving motor by control speed controller can make speed of rotate transformer constant, so as to compensate frequency difference of the two power networks and keep system balanced.