AbstractTo investigate what the minimum tritium inventory is for starting up a fusion experimental reactor of 143 MW-fusion power, a dynamic subsystem model of tritium fuel cycle system and mean residence time method are utilized to simulate tritium fuel cycle system of the fusion experimental breeder (FEB), in which liquid lithium (LLi) is used as tritium breeder and helium as coolant. In FEB design, the plasma burn-up fraction β=2.08% and tritium-breeding ratio Λ=1.10 are calculated. Results show the required minimum tritium storage is about 319 g to start up and operate FEB.