Fast isolating switch is one of the key components of Hybrid DC circuit breaker. It is difficult for conventional breakers to provide such a fast operating speed when the switching time is within a few milliseconds. In this paper, based on the study of the vacuum switch, focusing on the application of small vacuum gap in DC isolation, arcless breaking situation, a series of experiments were designed. After solving the problems of synchronous as well as voltage distribution in every break, a 200 kV DC fast isolating switch with less than 3 ms switching time is developed, and through the whole test of DC circuit breaker.