Abstract: Aiming at the problem that the traditional voltage withstand test requires bus power outage, expands the range of power outage, increases the risk of power grid, and is not conducive to the expansion and capacity increase of gas insulated switchgear(GIS) substation, based on the basic principle and technical difficulties of the same frequency and same phase AC voltage withstand test, the phase - locked loop(PLL) software algorithm is optimized to improve the frequency locking speed and phase locking accuracy, and the same frequency and phase voltage withstand test technology is improved in this paper. By accurately controlling the frequency and phase of the voltage, the voltage withstand test under the condition of incomplete bus stop is realized. At the same time, the successful application of this technology in GIS equipment expansion project is introduced.