The invention discloses a measuring and calculating method of GIS high voltage test through multiphase parallel connection before the operation of a transformer station, which resolves the problem that three repetitive wiring causes high labor intensity and high risks before the operation of the transformer station. A-phase GIS, B-phase GIS, and C-phase GIS are connected in parallel, and the measuring and calculating of the high voltage test are conducted. Based on the voltage resistance and frequency relation curve UC-Omega after the parallel connection of three phase GIS, if the borne pressure-resistant value is the maximum voltage resistance value, which is greater than or equal to the test pressure-resistant value of GIS release regulation, and Omega0 is greater than the limited frequency regulated by the A-phase GIS, the B-phase GIS, and the C-phase GIS, the three phase GIS can be connected in parallel and the high voltage test can be conducted. The invention is advantageous in that the AC voltage resistance test can be carried out smoothly, and test time can be saved.