Surface charge of insulator has the characteristic of nonuniform distribution. In particular, the electric field can be severely distorted by a surge of surface charges. Thus, the insulation reliability of DC gas-insulated transmission lines (GILs) is threatened. In this paper, the online measurement method and mechanism for surface charge surge is proposed based on the Kelvin electrostatic probe. The online measurement platform for surface charge surge is constructed. The effectiveness of the measurement mechanism is validated. The effects of initial surface charge are analyzed. The results indicate that it is easy to lead to a sudden change in the surface potential when the insulator surface charge sharply increases. Furthermore, there is dissipation of the accumulated surface charge. Once the surface charge is surged, there is a rapid decrease in surface potential. The decay rate of the surface potential is positively related to the initial surface charge. Therefore, the surface potential is affected by the surface charge surge. It is inferred that the insulator surface charge surge can be characterized based on the change in surface potential at the pouring port of basin insulators measured by the Kelvin electrostatic probe. The work in this paper provides a reference for online measurement of the surface charge surge on GIL insulators, improving the reliability of DC GIL insulators by online monitoring to avoid surface flashover.