The invention discloses an extra-high voltage GIS major loop insulation test electric quantity and test capability calculating method. A test capability estimation method of a frequency-conversion series resonance device used for an extra-high voltage GIS major loop insulation test comprises connecting the frequency-conversion series resonance device with an extra-high voltage GIS device in series; calculating a first test capability scope of the frequency-conversion series resonance device by taking regard of the capacity limitation of the frequency-conversion series resonance device and based on the relationship between the current through the frequency-conversion series resonance device and the electric quantity of the extra-high voltage GIS device; calculating a second test capability scope of the frequency-conversion series resonance device by taking regard of the frequency limitation of the extra-high voltage GIS device and based on the relationship between the resonant frequency of the frequency-conversion series resonance device and the electric quantity of the extra-high voltage GIS device; calculating the intersection set of the first test capability scope and the second test capability scope of the frequency-conversion series resonance device, and obtaining the actual test capability scope of the frequency-conversion series resonance device.