Long pulse high power pulse driver in the microsecond range is an important platform for conducting high power microwave (HPM) device studies and one of the key technologies is to prohibit surface flashover of the vacuum diode interface under the conditions of long pulse duration and the presence of guiding magnetic field. We report a design of a vacuum interface which works in the microsecond timescale with guiding magnetic field. Three measures are adopted to suppress the surface flashover. First is a cathode electron beam blocker, which intercepts the electron beam from the cathode and electron beam drift tube; Second is a grounded plate field shaper, which makes the electric field equipotential line and the interface form an angle of about 45°, so that the electrons emitted from the triple junction are directed away from the insulator; Third is a floating metal ring, which prevents the electron multiplication if emission from cathode stalk triple junction does occur. The electric field and magnetic field in the diode and the trajectory of electron beam are calculated, and the geometry of the vacuum interface is optimized. Experiments show that the vacuum interface of the diode can work at voltage of 400 kV and pulse width of 800 ns, which allows the study of long pulse high power microwave tubes.