Abstract: The molten salt reactor (MSR) depletion calculation model was studied, considering the cycling of fuel salt in the primary loop and isotope feeding and exaction through on-line processing system. Fuel cycle calculation function was developed and tested based on lattice code SONG. Fuel cycle and start-up schemes were analyzed for different core types, including BeO moderated molten salt thermal reactor and no moderator molten salt fast reactor. According to the reactivity requirement for long-term operation, corresponding on-line reprocessing and fissile isotope feeding schemes were analyzed for ²³³U and Pu start-up schemes. By the balance calculations of isotope feeding, exaction and density in the fuel salt, the influences of on-line processing and start-up schemes on Th-U fuel cycle efficiency were revealed, and technology roadmap was proposed preliminarily for molten salt reactor fuel cycle. The results show that using Pu from PWR is better than using ²³³U for start-up of Th-U fuel cycle, since Pu started fast spectrum molten salt reactor has much higher ²³³U production rate than ²³³U started reactors. However, when there is enough ²³³U accumulation, ²³³U started thermal spectrum reactor is preferred, which has a shorter doubling time and much less fuel loading.