The invention discloses a method utilizing an isothermy relaxation current method for evaluating an insulated overhead conductor, and belongs to the field of testing. The method comprises a numerical control direct current high-voltage power source, a numerical control micro current meter and a control computer. A layer of aluminum foil is arranged outside the insulated overhead conductor. A high-voltage output end of the direct current high-voltage power source is connected with a cable core of the insulated overhead conductor. A high-voltage output end of the numerical control micro current meter is connected with the aluminum foil. The numerical control direct current high-voltage power source and a low-voltage end of the numerical control micro current meter are grounded. A specific high-voltage polarization electric field is exerted on the insulated overhead conductor in advance under normal temperature, after polarization for a period of time, the high-voltage polarization electric field can be removed, and subsequent depolarization relaxation current of an insulating layer of the insulated overhead conductor can be detected. Due to the fact that the depolarization relaxation current comprises abundant insulating material microscopic performance information, parameters such as relevant microscopic parameters, dielectric loss and aging factors of insulating materials can be obtained through relevant analysis, and reliable basis is provided for insulating state evaluation of the insulated overhead conductor.