Kerr effect is defined as an electro-optical physical phenomenon, in which the testing laser can change its polarization state as passing through a Kerr medium stressed by applied field. In this work, an on-line measurement system based on Kerr effect is established to investigate the surface electric field of insulator stressed by nanosecond HV pulse in vacuum which include a HV pulse source, YAG laser, synchronous controlling apparatus, Kerr cell & insulator sample, optical phase shift detecting subsystem. According to comparison experiments, the distortion of insulator surface electric field caused by surface charging is observed. And a time-resolution measurement on this field distortion is also performed.