This paper proposes a method for the measurement of the large ground grid impedance. It is based on analysis of principles and combining with the advantages of the straight-line method and non-power-frequency method. It investigates how to put forward the voltage and current poles as well as choice the appropriate pilot frequencies. A hydropower station ground impedance has been measured and the results verify the correctness and applicability of the method. This paper concludes some problems that should be noted. It can provide references for ground impedance measurement.