Multi-terminal direct current（MTDC）transmission based on flexible direct current（DC）and DC grid technology are one of the most effective means for the interconnection and accommodation of renewable energy generation.The high-voltage DC circuit breaker is a key component in the DC grid.On the basis of an analysis of the mechanism of DC-side short circuit in the MTDC system and the trend of the corresponding short-circuit current，the characteristics of the fault current through a DC bus of Zhoushan5-terminal flexible DC transmission project under the most severe short-circuit fault are detailed，and the demands for the DC circuit breaker in the DC grid are stated.After a performance comparison of the three types of DC circuit breakers in current use，a novel topology of fast DC circuit breaker suitable for use in the flexible DC transmission grid is proposed.Simulation results have verified the fast action of the novel DC circuit breaker of cutting off the fault current of the flexible DC transmission grid.