In order to meet the requirements of ultra-high-speed protection action of multi-terminal flexible DC grid transmission lines, based on the traditional DC engineering line protection, the protection configuration and action strategy of flexible DC line are introduced.According to the characteristics of fault traveling wave, a traveling wave protection method is proposed.The method extracts the specific frequency band characteristics of the fault wave by S-transformation, and identify the ground short circuit fault by changing the amplitude of the fault traveling wave at different times, thereby forming a criterion for the traveling wave protection exit.Finally, based on the actual engineering parameters, the multi-terminal flexible DC simulation model built by RTDS and the closed-loop system composed of control and protection system are tested.The test results show that the method is effective and reliable.