Abstract: The content of ²³⁶Pu in the ²³⁸Pu product is limited in the range of 2 ppm to protect the workers during the manufacture of the heat source. Therefore, the content must be reduced as much as possible in the production process, and the ²³⁶Pu content index must be strictly controlled. The control of plutonium content is an important parameter of plutonium heat source, studied the conditions for accurate measurement of trace ²³⁶Pu in irradiated Np target. A separation method for Pu in the solution of irradiated Np target was established. According to the composition of solution, TBP-TEVA double extraction chromatographic column separation was adopted. The plutonium valence was adjusted by FS sulfonic acid and sodium nitrite, and the impurities of Al, Fe, U, Th and Np in the plutonium were decontaminated, the decontamination coefficient was greater than 10⁴, and the recovery of plutonium was 90.7%. In this paper, the interference of a large amount of ²³⁸Pu on ²³⁶Pu was studied, and the results showed that a large amount of ²³⁸Pu would cause increasing of the background, and coarsening of the resolution of ²³⁸Pu peak and increase the MDA. The MDA of ²³⁶Pu is 1.20×10^-2 Bq and the isotope ratios of n(²³⁶Pu/²³⁸Pu) is 4.63×10^-8, when the ²³⁸Pu is 7 500 Bq background interference and measure it for 4.3 h. When the isotope ratios of n(²³⁶Pu/²³⁸Pu) is more than or equal 4.63×10^-8 in the sample of irradiation Np target, meanwhile sampling the suitable amount can make the ²³⁸Pu activity in the electroplating source range from 450-7 500 Bq, then the trace ²³⁶Pu can be detected, and the ratios of n( ²³⁶Pu/²³⁸Pu) can be accurately measured. The determination of trace ²³⁶Pu in ²³⁸Pu heat resource is an important technical indicator in neptunium target irradiation process and product of heat source. In this study, the combined TBP-TEVA resin was used to separate and purify the plutonium from the irradiated neptunium target solution, and the alpha energy spectrum method was used to analyze it.