The existing online cable insulation condition (CIC) evaluation methods are mainly based on the insulation defects signals inside the cable, such as dc signals, or the transient disturbances from outside, such as ground faults. However, the former are usually weak and susceptible to environments, and the latter are not frequently obtainable. On the contrary, the steady-state disturbances, such as harmonics, which carry CIC information, have been widely measured and are readily available. Therefore, it is very convenient if we can use these data to evaluate CIC. In fact, based on the harmonic measurements at one end of a cable and the cable parameters corresponding to any CIC, the theoretical harmonic amplification factor (HAF) can be obtained, and the HAF is the ratio of harmonic values at both ends. In other words, if there is a deviation between the measured and theoretical HAF, it means that CIC has changed. Given this, we propose a novel method to evaluate CIC by minimizing the deviation, which includes three steps: 1) derive the theoretical HAF by using the cable parameters and harmonic measurements at one end of a cable; 2) construct the objective function to minimize the deviation; and 3) solve the function to evaluate CIC by employing the Gauss-Newton iteration algorithm. The performance and effectiveness of the proposed method are verified through the simulation and field cases.