In the error check of electromagnetic potential transformer in gas insulated metal enclosed transmission line (GIL), the selection and combination of resonant power system are key problems. Consequently, this paper introduced the use of combined adjustable reactor to solve the voltage boosting in GIL, and analyzed the mathematical model of the system. The concept of relative detuning at power frequency resonance was presented. the factors which restricted the voltage gain were also analyzed. In addition, the use of secondary voltage drop tester for error checking was proposed. The key parameters measurement and test steps were given. Finally, taking the error test in GIL electromagnetic voltage transformers of Nuozhadu hydropower station as an example, experiment results have been given to verify theSfeasibility and validitySof this method