Broadband impedance spectroscopy (BIS) has become a new method for detecting cable insulation defects, but the sensitivity of this method to local defects in cables has not been fully studied. Taking long submarine cable as an example, firstly, the intrinsic correlations between the impedance spectroscopy characteristics and the cable parameters are studied, and the influence of local defects on the peak of impedance spectrum is studied by establishing a simplified model of long cables with insulation defects. Secondly, the formula for calculating the sensitivity of input impedance to the insulation defects is proposed and the characteristics of sensitivity are studied. Finally, the sensitivity of the input impedance to different positions and degrees of defects at the first resonant frequency is analyzed, which can be verified by simulation and experimental. The Results show that, for long cable with length l, under the condition of open-circuit, the sensitivity trends to zero when the defects occur at 25% or 75% of the cable length l, and under the condition of short-circuit, the sensitivity trends to zero when the defects occur to the end of the cable. Therefore, in the application of BIS, the above positions should be paid attention to, and the cable length can be changed and measured again to confirm its insulation status. INDEX TERMS Submarine cable, distributed parameter circuits, cable insulation, broadband impedance spectroscopy (BIS), sensitivity, the resonant frequency.