The invention discloses an equipment and method utilizing an isothermal relaxation current to evaluate an aging state of a fluorescence fiber temperature sensor sheath in a transformer. The equipment comprises a clamp used for fixing and clamping fiber and an isothermal relaxation current measurement and control system; according to the method for evaluating the aging state of the fluorescence fiber temperature sensor sheath in the transformer, a special high voltage polarization electric field is pre-applied to the fiber, after polarization for a time segment, the high voltage polarization electric field is removed, and a depolarization relaxation current is detected; microscopic parameters related to insulation materials and aging factor parameters can be acquired through relevant analysis, and reliable bases are provided for fluorescence fiber sheath insulation state evaluation. The test equipment comprises a DC high voltage source and a microammeter, and voltage output and current detection precision is high; the rich material microscopic performance information in the isothermal relaxation current is utilized, a new surface electrode distribution mode is proposed, and relevant analysis theories are combined; and the test system has a light structure and is convenient to mount and carry.