Abstract: Surface flashover phenomena in vacuum often occur along the insulator,with its onset flashover voltage much lower than the breakdown voltage of either the insulator or the vacuum gap with the same length,so it seriously limits the development of high voltage electro-vacuum devices.Introducing a kind of machinable ceramic（MC） with excellent machinable performance and good surface dielectrical capability into vacuum insulation system,and considering the commonly used insulation stack structure,multiple layers machinable ceramic samples were established.Their surface flashover voltage were investigated under nanosecond pulsed voltage in vacuum,and the surface electric field distributions and electron trajectory in different structure were simulated.The experimental results indicate that,the flashover voltage of multiple layer insulators is better than that of the cylinder,and it improves with the increase of ratio of insulation to metal（I/M）,the radial electric field of multiple layer insulators decreases with the increase of I/M.The flashover stability of truncated cone MC multiple layer insulators is better than that of truncated cone MC with the same angle,and the electrons are not easy to impact the surface of samples.