A multi-step Monte-Carlo method is established for calculating the γ spectrum of plutonium volume source. The γ spectra of a spherical plutonium source is calculated by using a multi-step and direct mothed,and compared with the expe rimental γ spectrum. The calculated results are in agreement with the experm ental results whithin 10%. It is proven by the calculated result of Steve Fett er model that the multi-step calculation converges faster than the direct metho d in the calculation of the γ　spectrum of plutonium volume source with shields .