Abstract: The safety of nuclear fuel storage, transportation, and post-processing is important component of nuclear and radiation safety. The envelope source term should be obtained to ensure safety of storage and transportation of spent fuel assemblies and enhance economic performance of spent fuel transportation, meanwhile, to reduce environmental radioactivity release of post-processing plant. The ORIGEN-ARP program was utilized to analyze the impact of power history, uranium enrichment and burnup on assembly source terms. Power history has some effect on source term, but an appropriate envelope factor can be adopted on cover it, and this impact can also be ignored after certain cooling time. Calculation shows that the source term of spent fuel assembly which with enrichment of 4.45% and burnup of 55 GW•d/tU is enveloped among current spent fuel assembly, and can be used as initial source term for design of spent fuel and shipping cask and analysis of environmental radioactivity release.