Frequency domain reflection (FDR) has been usually applied to detect power cable defects. Nevertheless, almost all of the existing FDR methods are offline. Thus, in this work, a capacitor-resistor (CR) based circuit is proposed to help achieve online FDR. The proposed method is to use high-frequency passing and low-frequency blocking characteristics of the capacitor as the main isolator and use a resister to ensure the port voltage of the circuit is in a certain safe range. Nevertheless, the C-R circuit also has some negative impacts on the FDR cable defect test result, such as extra impedance accumulations and mismatching. Thus, the impact of the C-R isolation circuit with a certain value of capacitor and resistor on the test result is analyzed. Through the theoretical analysis, a recommended isolation capacitor is slightly larger than 2 nF, and a recommended isolation pull-down resistor equals the characteristic impedance of the defect power cable. The simulation results and experiment results indicate the proposed method is effective.