The harmonic current will cause the loss of the distribution transformer, the acceleration of the thermal aging and the decrease of the insulation life. A method for calculating transformer core loss under harmonic current disturbance is proposed. The distribution transformer of Yyn0 and Dyn11 connection of the stray loss in harmonic current disturbance been analyzed. So the calculation method of transformer load loss based on harmonic loss factor is improved, and corrected hot spot temperature calculation model. The calculation method of relative aging factor of insulating paper in IEEE C57.91 was modified by using 6℃ criterion, the reference value of transformer hot spot temperature at 98℃. The results of calculation and experiment show that the transformer loss increases with the increase of the harmonic current distortion rate, so the life loss of transformer insulation system is shortened, and the service life of transformer is shortened.