Surface flashover is an important accident threating the safe operation of gas-insulated switchgear (GIS). Non-invasive surface potential measurement for flashover test device with finger-shape electrodes is proposed,and the feasibility of this device is analyzed by finite element software. The results show that non-invasive measurement error is 2.42% under simulation parameter settings in this paper. Surface potential of epoxy with nano-SiO₂ mass fraction of 2% is measured using this device and the ascending applied voltage step is 0.5 kV/5 min. When the applied voltage is unable to induce flashover phenomenon,Maxwell-Wagner interface polarization caused by nanoparticle addition causes surface potential decay process. When the applied voltage approaches flashover voltage,surface potential presents periodic rise phenomenon in the decay process and the frequency of rise phenomenon becomes faster as the applied voltage higher. Periodic rise phenomenon in surface potential decay process before flashover can be used as a signal for predicting surface flashover. Fluctuation of surface potential in the continuous flashover process is smaller than that of intermittent flashover and the interval between adjacent flashover becomes shorter,which is related to the fact that surface burning path in the continuous flashover is prone to flashover. Surface potential measurement of DC flashover provides guidance for flashover prevention and flashover severity evaluation.