There is a difference between the actual impedance of substations’ ground grid under harmonic interference and that measured under power frequency. Especially under strong harmonic interference, the difference becomes more obvious between the actual one and that measured under power frequency. A method is proposed in this paper to calculate parameters of ground grid models by measuring the ground grid impedances under power frequency and then calculate those under harmonic interference. This method is capable of solving the problem that it is impossible to measure the actual grounding impedance of substations operating under harmonic waves. The feasibility of the method is verified by means of simulating calculations. After correcting by this method, the grounding impedance modulus error becomes less than 2% and the impedance angle error less than 10%. This method may be applied to the measurement and analysis of grounding impedances of substations under harmonic interference.