Polybutylene terephthalate(PBT) was employed as test sample to investigate the characteristics of surface charge migration. The experiment was carried out on sample surface under a negative dc stress between two parallel electrodes. An electrostatic probe was designed to measure the surface charge density with minimum charge resolution of 0.018 pC. Obtained results show that negative charges appear near high voltage electrode, while positive charges emerge in front of grounded electrode. The hetero charges migrate towards the opposite electrode with time, forming a junction line near the grounded electrode. The polarity of surface charge tends to be positive dominated with the lapse of decay time. It is suggested that the local field at HV electrode will be seriously enhanced if the source is turned on again.