Abstract: A γ-ray spectrum analysis algorithms of multi-overlap-peak model and Levenberg-Marquardt (an improved nonlinear least squares fitting) for ²⁴⁰Pu abundance were developed to achieve accurate results. The peak area calculation can avoid the risks of associated subtraction by multi-peak fitting directly. The experiment results show that the improved method can give better results of ²⁴⁰Pu abundance, compared with the previous algorithm. The uncertainty of ²⁴⁰Pu abundance is less than 10% at 95% confidence level.