This paper describes a method to design an external controller for secondary voltage controller (SVC) and tune both controllers in order to improve transient stability with real-time simulation. Recently, Korea Electric Power Corporation (KEPCO) has developed a novel voltage management system (VMS), a kind of SVCs, and installed in Jeju Island. It has been operated successfully to balance reactive power reserve of the controlled generators and enhance the voltage stability. However, the VMS has an inherent demerit in short-term recovery after a disturbance such as load change and tripping transmission line occurs. In this paper, after having a discussion related to the demerit in detail, an external controller is proposed. Also, a method to tune the external controller for reducing the bad reaction of the VMS is implemented with real-time digital simulator (RTDS) which is a fully digital power system simulator in real-time. The performance is evaluated by time-domain simulation.