Abstract: Molten salt nuclear reactor (MSR) uses graphite as a neutron moderator or reflector, and the graphite is directly contacted with the molten salt. The irradiation stability of the graphite and chemical reaction corrosion between the graphite and salt is the critical researches to be studied. Microstructure, crystal structure, chemical bonding and property of the ionimplanted finegrained isotropic graphite under different irradiation doses were studied by using the 3.0 MeV He+ simulation. The results show that under high temperature, the increase of the defect concentration and surface morphology change are much less than those under the room temperature because of the high temperature annealing. The defect concentration decreases under the irradiation and molten salt environment. The improvement of microstructure is related to the annealing effect in high temperature molten salt environment and the closure of internal microcracks caused by molten salt solidification. The C—F bond could be detected by the long time molten salt immersion, which is closely related to the defect concentration and types. Vacancy cluster and migration of the interstitial atom affect the formation of the graphite intercalation compounds. It is all not avail to forming the C—F bonds, which decreases the destructive effect of the graphite surface structure.