The present invention relates to an oil-immersed transformer overtemperature warning method under a harmonic condition. The method comprises a step of determining any harmonic current distribution and the transformer winding eddy current loss, transformer stray loss and transformer total load loss actual values of a same total current effective value, a step of determining the relation among a transformer load loss, a harmonic distortion rate and a load rate, a step of determining the relation among a transformer hottest point temperature rise, the harmonic distortion rate and the load rate, and a step of determining the threshold of a corresponding alert level. According to the method, through transforming the basic formula of the transformer harmonic loss, the relation among the harmonic loss, a harmonic current distortion rate and the load rate is obtained, thus according to the relation model between transformer temperature rise and loss, the temperature rise condition of a transformer after a certain time is derived, and the early warning of the transformer temperature rise is realized.