Abstract: Harmonic resonance has frequently occurred in electric railway systems due to the match between injected harmonic currents and system resonance points. Impedance characteristic can accurately reflect the system resonance point, thus the accurate impedance measurement for electric railway is needed. This paper proposed a method for measuring the broadband impedance of traction power system at controlled frequency band based on active harmonic excitation. A chirp pulse width modulation（chirp-PWM） signal was adopted to drive the large-power harmonic excitation circuit, and the harmonic excitation at controlled frequency band can be obtained by only once disturbance. Then, the impedance at controlled frequency band can be calculated according to the response voltage and current. This method has less measurement time compared with the traditional frequencysweeping method. Meanwhile, the structure of harmonic excitation circuit is simple and the control signal can be obtained easily. The measurement is more flexible due that the measured frequency band can be controlled, which can be an important reference for designing the impedance measurement equipment of traction network.