Abstract: An on-line measurement system based on Kerr effect is established to investigate the surface electric field of insulators stressed by nanosecond high-voltage (HV) pulse in vacuum. A 180 kV Marx generator with the high-voltage cable as the pulse forming line are utilized to output a square nanosecond HV pulse stressing on insulator specimen in Kerr cell, once a gas switch is conducted by a laser pulse emitted from a YAG laser. And a synchronous controlling apparatus is developed to make sure that the measurement on surface electric field is performed during the HV pulse being stressing on the insulator. Accordingly, a preliminary experimental observation on surface electric field is performed.