With advancements in cable manufacturing processes, the physical parameters of certain cable conductors fall outside of the scope specified by the IEC60287-1-1 standard, and the alternating current (AC) resistance calculated using the IEC standard may lead to reliability issues in the thermal evaluation of cable lines. Therefore, conducting an AC resistance test on cable conductors becomes critical for the thermal evaluation of cable lines. The source of error in the existing AC resistance test was analyzed first. It was found that the characteristics of the source used in the test lead to an error between the test value and the actual value of AC resistance. Moreover, an optimized AC resistance testing method based on active power was proposed to decrease the error. The accuracy of the method was also demonstrated. Finally, AC resistance tests were conducted on cable conductors with different cross-sectional areas, segmental methods, and oxidation methods by using the proposed method. The test results are also thoroughly discussed.