Lithium is an attractive coolant for space nuclear reactors due to its good thermal properties and low density. The main objective of this study is to investigate the effect of coolant Li-6 concentration on the neutronic parameters of the lithium-cooled space reactor, with the aim to provide an appropriate reference for the purification of lithium coolant and the safety of the lithium-cooled space reactor. The neutronic calculations based on the lithium-cooled reactor are performed using the SuperMC code with the ENDF/B-VII cross-section database. The effects of coolant Li-6 concentrationwere studied over the range of 0 to 8.5 at.% as well as the corresponding effects on the depletion, heliumproduction, neutron spectrum, and temperature reactivity coefficient. The results show that the Li-6 concentration of 0.01 at. % in the lithium coolant is appropriate in the lithium-cooled reactor. In this case, the neutronic parameters including the depletion, helium production, and neutron spectrum showed no obvious change compared to that of the pure Li-7, and the coolant reactivity coefficient has a negative effect on reactivity.