Inventories of radioactivity in Xi'an Pulsed Reactor (XAPR) are calculated, with the assumption that XAPR is continuously or intermittently operated at full power of 2MW until the fuel rods archive the design burnup. With a conservative releasing model, the biggest inventory 4.13×1016 Bq was selected to evaluate the source term of XAPR in an extreme severe accident, in which all fuel rods are supposed to be ruptured. The result shows that up to 41.0% of radioactive fission products is released in 1 minute, the release lasts for about 5 minutes, and the total radioactivity released to environment is 4.54×1012 Bq. ©, 2014, Yuan Zi Neng Chuban She. All right reserved.