Channel characterization measurements of power line are often carried out when the power system is operation actively. So the coupling device is adopted to isolate the power frequency signal. Due to the impedance mismatch between the output of coupling device and power network, the energy of carrier signal will be attenuated, which causes much influence on the accuracy of measurement results. In this paper, it analyzes the error introduced by the coupling device for network impedance measurement, noise measurement and path loss measurement. A practical algorithm is proposed to correct the measurement deviation. It also points out that in path loss measurement, using the arbitrary waveform generator associated with spectrum analyzer will eliminate the influence of impedance mismatch automatically.