The calculation method to calculate transformer losses with the current distortion is given in IEEE Std C57.110, it use the transformers harmonic eddy current loss factor and harmonic spurious losses factor to calculate transformer eddy current losses and spurious losses, but the influence of skin effect and proximity effect on transformer’s winding resistance not are considered,so the result is not accurate. the harmonic loss factor for winding resistance losses is introduced in the paper, to accurate calculate transformer resistance losses and the maximum load current. On the basis of the maximum load current, Study of relationshio of the current distortion rate and dry-type transformers derating rate . The results show that harmonic ratio has great influence on dry-type transformers maximum load current and load capacity, the load capacity is reduced 50% when the harmonic distortion rate reached 60% .