The invention relates to a method and a device for measuring harmonic impedance. Aiming at the main error source in an 'incremental method' of the method for measuring the harmonic impedance, namely the problem of the synchronization of frequency deviation and two different time signal phase angles, the invention discloses a 'bisynchronous incremental method', and develops a corresponding testing instrument, wherein the first synchronization is that a reference phase is synchronized with the system cycle to solve the drifting problem of the reference phase; and the second synchronization is that the sampling frequency is synchronized with the system frequency by tracking the changes of the system frequency by a hardware phase lock circuit and generating the sampling frequency by the phase lock circuit. The device for measuring the harmonic impedance comprises a synchronous circuit of the sampling frequency and the system frequency and a synchronous circuit of the phase and the system cycle. The 'bisynchronous incremental method' solves the error problems caused by benchmark reference phases, system frequency fluctuations and the like in the 'incremental method'; the device has a simple principle and is easy to implement; and the method and the device reach the engineering application level and solve the problem of measuring the harmonic impedance in the generation of electricity.