Abstract: The calculation model of radioactive activated source term for decommissioned reactor was set up, and the validity of the model was verified by critical calculation. The significance of layering calculation and stepping calculation for the area far from the reactor core was analyzed by MCNP code. Using the combination of MCNP and ORIGEN codes, the specific activities of ⁶⁰Co of reactor core components, such as the reactor support skirt, the samples of the inner and outer steel plates of the lead support barrel and the pre-installed sample on the outer part of the primary water barrel, were calculated specifically. The deviation between the calculation value and measurement value meets the engineering design requirements, and this shows that the model and the calculation method for calculating the radioactive activated source term for such a decommissioned reactor are adequate.