Abstract: ADS sub-critical experimental assembly is the first decommissioning sub-critical assembly, by investigating the decommissioned radioactive sources of which, a method which is applicable to the decommissioning of zero-power facilities was developed. First, a detailed investigation of the radioactive sources in the ADS sub-critical experimental assembly was conducted. Then, the method combining the ORIGEN and Monte Carlo codes was used to calculate the radioactive nuclides, source strength, and specific activity generated by each component of the reactor core. Finally, through on-site measurements and sample analysis, the radioactive sources of the reactor core structural components were determined, the results can be used as a reference for the source item investigation of other zero-power facilities.