The molten salt fast reactor(MSFR) shows great promise with high breeding ratio(BR),large negative temperature coefficient of reactivity,high thermal-electric conversion efficiency,inherent safety,and online reprocessing.Based on an improved MSFR optimized by adding axial fertile salt and a graphite reflector,the influences of ~7Li enrichment on Th-U breeding are investigated,aiming to provide a feasible selection for the molten salt with high fissile breeding and a relatively low technology requirement for ~7Li concentration.With the self-developed molten salt reactor reprocessing sequence based on SCALE6.1,the burn-up calculations with online reprocessing are carried out.Parameters are explored including BR,~(233)U production,double time(DT),spectrum,~6Li inventory,neutron absorption,and the tritium production.The results show that the Li enrichment of 99.95% is appropriate in the fast fission reactor.In this case,BR above 1.10 can be achieved for a long time,corresponding to the ~(233)U production of130 kg per year and DT of 36 years.After 80 years’ operation,the tritium production for 99.5% is only about 7kg,and there is no obvious increase compared to that for 99.9995%. The molten salt fast reactor (MSFR) shows great promise with high breeding ratio (BR), large negative temperature coefficient of reactivity, high thermal-electric conversion efficiency, inherent safety, and online rep.Bing on an improved MSFR optimized by adding axial fertile salt and a graphite reflector, the influences of ~ 7Li enrichment on Th-U breeding are investigated, aiming to provide a feasible selection for the molten salt with high fissile breeding and a relatively low technology requirement for ~ 7Li concentration .With the self-developed molten salt reactor reprocessing sequence based on SCALE 6.1, the burn-up calculations with online reprocessing are carried out. Parameters are explored including BR, ~ (233) U production, double time (DT), spectrum, ~ 6Li inventory, neutron absorption , and the tritium production. The results show that the Li enrichment of 99.95% is appropriate in the fast fission reactor. In this case, BR above 1.10 can be achieved for a long time, corresponding to the ~ (2 33) U production of 130 kg per year and DT of 36 years. After 80 years’ operation, the tritium production for 99.5% is only about 7 kg, and there is no obvious increase compared to that for 99.9995%.