Abstract: Frequency domain reflectometry (FDR) is an effective method for the cable insulation condition diagnosis. Focusing on the problem of the impedance mismatching in the field FDR detection, this paper presents an effective FDR impedance matching technology. By introducing the variable series impedance model, combined with the estimated characteristic impedance, the influence of the bifurcated leads on the field FDR test results is effectively removed. Firstly, the cable model was established, and by the use of the principle of multiple refraction and reflection of the traveling waves, the reflection coefficient at the head of cable was deduced, which explained the signal transmission process in the cable. Then, this paper presented an impedance matching method at the head of the cable when the bifurcated lead was existed. Finally, the impedance matching tests were performed on the coaxial cable and the 10 kV power cable, and the validity and accuracy of the method in this paper were verified. The results show that the proposed method is able to effectively solve the problem of impedance mismatching at the head of the cable in the field FDR detection. By using the matched data, the attenuation of the cable is able to be compensated effectively, and the phase velocity of cable be calculated accurately.