Abstract: Two bracing type MV gas switches are designed, the cantilever switch has a cantilevered HV electrode sealed in the brace insulating cylinder, the section-brace switch has an HV electrode spreading out of the brace insulating cylinder. Through self-breakdown experiment and electrostatic field simulation, the optimization of insulator envelop is discussed. The result indicates that the section-brace switch is safer and stabler than the cantilever switch. The reason of puncture in insulator cylinder of cantilever switch is that the stream formed in switch is accelerated by the electric field and then strikes the surface of outer cylinder. Bradley formula with field enhancement factor is given and it accords well with experimental data.