Harmonic loss is one of the main causes of safety accidents in excitation transformers. If the loss value is too large，it can easily affect normal substation and transmission. Therefore，a method based on wavelet packet decomposition and reconstruction was proposed to calculate the additional loss of rectification harmonics in excitation transformers. By obtaining wavelet packet decomposition and reconstruction coefficients，the additional losses of two types of harmonics in excitation transformers were analyzed. The open circuit and short circuit experiments were used to obtain the equivalent circuit parameters of the excitation transformer，and the wavelet packet decomposition and reconstruction algorithm was used to calculate the resistance and reactance under different harmonic frequencies，and compare them with the reference value to obtain the additional loss value of the rectification harmonic of the excitation transformer. Finally，a certain type of excitation transformer was selected and used the proposed method to calculate its additional loss value under different harmonic frequencies. The results show that the calculated loss value is very close to the actual results，verifying the high practical value of the proposed method.