A molten chloride salt fast reactor (MCFR) is well suited to fuel breeding and the transmutation of transuranium (TRU) elements owing to its advantageous features of fast neutron spectrum and high TRU solubility. However, the neutron absorption cross section of 35Cl is approximately 1000 times greater than for 37Cl, which has a significant impact on the neutron physical characteristics of a MCFR. Based on an automatic online refueling and reprocessing procedure, the influences of 37Cl enrichment on neutron economy, breeding performance, and the pro-duction of harmful nuclides were analyzed. Results show that 37Cl enrichment strongly influences the neutron properties of a MCFR. With natural chlorine, 233U breedingcannot be achieved and the yields of S and 36 Cl are very high. Increasing the 37Cl enrichment to 97％brings a clear improvement in its neutronics property, making it almost equal to that corresponding to 100％ enrichment. More-over, when 37Cl is enriched to 99％, its neutronics param-eters are almost the same as for 100％ enrichment. Considering the enrichment cost and the neutron proper-ties, a 37Cl enrichment of 97％is recommended. Achieving an optimal neutronics performance requires 99％ 37Cl enrichment.