Abstract: In the Jiangsu Baihetan ±800 kV UHV hybrid DC project, when the three-phase ground fault traversal failure on the AC side of the receiving end occurs, a high amplitude overvoltage will be generated on the ±400 kV flex bus, which seriously threatens the safety of the flex manifolds, thus the ultra-fast bypass switch based on the plasma injection quickly grounding the flex bus within 1ms to protect the flex manifolds is a brand-new solution. The DC 400 kV trigger gap needs to meet the stringent requirements of high withstand voltage (> 1.2 MV), low operating coefficient (< 10%), trigger delay (< 1 ms), and large through-flow. The key to problems is how to realize the reliable trigger conduction with high air pressure and long gap and to put forward the engineered design scheme. In this paper, a research platform of the trigger gap based on plasma injection was built to research the influence of key factors such as trigger voltage, air pressure, gap length, and operating coefficient on the conduction characteristics of the SF₆ trigger gap triggered by enhanced plasma jet. The results show that increasing the trigger voltage can significantly improve the conduction probability and reduce the trigger delay, but there is a certain saturation phenomenon. At a fixed operating voltage, with the increase of air pressure or gap length, the working coefficient decreases significantly, resulting in a sudden increase in the trigger delay and dramatic decrease in the conduction probability. At the same working coefficient, the conduction characteristics of the short gap with high air pressure is stronger. Accordingly, it is proposed to meet the extra-high-voltage hybrid DC project 400 kV flexible bus rapid grounding with DC400 kV gap switch, to adopt the gap with a composite jacket single-gap structure, and to adopt a rotating trigger cavity design to achieve multiple triggering. A trigger gap test prototype (gap distance 10 cm, 0.5 MPa SF₆) is designed, of which the lightning withstand voltage is greater than 1250 kV, the critical trigger voltage is lower than 50 kV, and the trigger delay is shorter than 0.5 ms. The research results provide a theoretical support and preliminary engineering design scheme for the engineering application of megavolt SF₆ trigger gap for fast protection of flexible DC valve group in UHV hybrid DC transmission system.