In the analysis of nuclear energy spectrum, the single energy peak is affected by low energy tail and high energy tail. Therefore, a method is proposed in which the data fitting window is determined by the full width at half maxima values of single energy peak, and then the Gaussian function fitting method is applied by using fast iterative algorithm. In this paper, the Gaussian signal fitting experiment with noise is carried out, then, the single energy peaks with different contents in the actual measured X-ray fluorescence spectrum are fitted by fast Gaussian method. The results show that this method has a good fitting effect on the single energy peak of high content nuclides, and the single energy peak of high background and low content nuclides should be smoothed before fitting.