The sudden change of power after large capacity flexible DC fault blocking will have a significant impact on the safe and stable operation of power grid. Because of the integration of flexible DC control and protection system, and the related technical parameters not being disclosed, study of fault electrical characteristics is based on Luxi flexible DC simulation system collocation with actual control and protection system. And then this paper proposes the criterions of single-phase grounding, phase to phase short circuit, phase to phase grounding, three-phase grounding, the positive and negative terminal grounding of the DC circuit, pole-to-pole short circuit fault and no fault tripping of valve side in converter transformer of VSC-HVDC system adapting to the security and stability control, and validates them in the RTDS simulation system. The results show that the proposed criterions can accurately identify the fault state of the two-terminal VSC-HVDC transmission system, so it has better engineering practicability. This work is supported by Science and Technology Project of State Grid Corporation of China “Research on Operation Characteristics and Control Technologies for ±500 kV VSC-HVDC System”.