Abstract: In order to suppress the influence of background harmonics on harmonic disturbance components and impedance measurement accuracy, a harmonic impedance measurement device based on cascaded H-bridge converter is studied. The topology of the harmonic impedance measuring device is studied, and the parameter design of each part of the harmonic injection device is given. In order to realize amplitude-frequency modulation of output signal and suppression of background harmonic interference, an improved control strategy of harmonic generating unit is proposed, which includes harmonic control and suppression: harmonic control sets the amplitude and frequency of output signal to realize amplitude-frequency control of output signal. Harmonic suppression is introduced to improve the power grid voltage feedforward control, which combines the first-order leading component of the power grid voltage feedforward with the capacitive current loop to suppress the interference of background harmonics to the output signal. Finally, Matlab/Simulink is used to verify the feasibility and effectiveness of the harmonic injection device and control strategy. Compared with the simulation results obtained without adding the harmonic suppression link, the harmonic impedance measurement error obtained by the design scheme in this paper is reduced by 1%, and the measurement error is no more than 0.5%.