For the 10 GW wind power bases in China, wind-thermal-bundled power transmitted by AC/DC system is regarded as one of feasible ways, but the stability characteristic of this kind of transmission system should be investigated. Angle is the special symbol to reflect the system transient stability, in the paper, the influence of angle rapidly changing behavior of DFIG and equivalent mechanical rotational inertias of wind-thermal-bundled system on synchronous generators angle transient stability characteristic are studied. To solve the problems of transient stability, an additional control strategy is proposed and is designed to become a control device prototype. Using real-time digital simulator (RTDS), the control device prototype is validated, simulation results show that the proposed additional control strategy can enhance the equivalent damping effect of DC system and DFIG to synchronous generators by changing the DC transmission power and adjusting the active power output of DFIG which can eliminate or reduce the imbalance of instant power in the transient process, as a result, the amplitude of angle fluctuation can be effectively inhibited, and the transient stability characteristic of wind-thermal-bundled power transmitted by AC/DC system is improved.