Abstract: According to the overlap of ²³⁸Pu and ²³⁹Pu+²⁴⁰Pu in alpha spectrum, a fitting extrapolation method was introduced to analyze peak counts in the alpha energy spectrum for calculation of two-peak radioactivity. The method is based on the tailed trend of main peak in spectrum that the counts in the tailed channels decrease with the decrease of channel number. The total counts of main peak were obtained by fitting extrapolation according to the sum counts of main peak with the change of the tailed channels. This method is simple and applicable, and the results in more than 30 alpha energy spectra in our work are with accuracy.