Abstract: A ultra high voltage（UHV） converter transformer was taken as the research object, whose type was ZZDFPZ-340800/500-800. Time-domain analysis and frequency-domain analysis were used to calculate the losses of converter transformer windings subject to non-sinusoidal load current, and obtained the detailed distribution of windings losses. Result comparison between two different methods has been done, and it could be found that frequency-domain analysis method was much more suitable for practical design. In order to overcome the shortcomings of GB/T 18494.2-2007 and IEC-61378 which ignored the effect of different skin depth on eddy losses of windings under the different frequencies, a harmonic losses factor was introduced to correct the losses, and to get a more accurate result. Using the corrected result to form a basis guarantee for the purpose of losses evaluation can reduce the manufacturing cost of the converter transformer.