Abstract: The gamma spectrum of ²³⁸Pu low energy photon source which was used for X-ray fluorescence analysis was analyzed by using P-type coaxial HPGe detector. There were characteristic rays of ²³³Pa, ²²⁴Ra, ²¹²Pb, ²¹²Bi and ²⁰⁸Tl in the spectrum except the characteristic gamma rays of ²³⁸Pu.²³³Pa was the decay product of ²³⁷Np, which was the raw material used to produce ²³⁸Pu. ²²⁴Ra, ²¹²Pb, ²¹²Bi and ²⁰⁸Tl were the daughter nuclei of ²³⁶Pu, which was by product in the process of ²³⁸Pu’s production. Gamma rays of 350, 440, 844, 1014, 1130, 1266, 1368, 1454 keV were caused by coulomb excitation that alpha particles bombarded encapsulation materials of the source, or nuclear excitation that gamma rays irradiated surroundings. After the basis of efficiency calibration, the radioactive activity of each radioactive nuclide was calculated by gamma ray spectrometry. The quality of each radioactive nuclide was calculated derived from the principle of radioactive equilibrium. Through the analysis of ²³⁸Pu’s gamma ray spectrum, the method to calculate the radioisotope activity and the purity of radioactive sources was offered.