In order to realize the rationality of the segmentation, equipment selection and parameter matching of the main circuit insulation test of 1100 kV Gas Insulated Switchgear (GIS), this paper presents an electrical parameter estimation method. First, the test method of main circuit insulation is given and the test principle and voltage program are outlined. Then, considering the limitation of test frequency and equipment capacity, the methods of estimating test capacity of the variable-frequency series resonant device with or without parallel connection are presented. Next, it proposes a method of how to estimate electrical variables of the Extra-High-Voltage (EHV) GIS main circuit insulation test at specified range. Finally, an example of EHV GIS main circuit insulation test is given to estimate the test capability of the series resonant device and to verify the accuracy of the electrical variables estimation method. The estimation method can accurately evaluate the test capability of the variable-frequency series resonant device, and realize the accurate estimation of the electrical parameters when specifying the test range, which provides a reliable reference for the segmentation and equipment selection of the EHV GIS main circuit insulation test. This work is supported by National High-tech R & D Program of China (863 Program) (No. 2015AA050204) and Science and Technology Project of State Grid Corporation of China (No. 520626150032).