Molecular dynamics simulation is applied to the investigation of the isotopic effects during a hydrogen isotope atom bombarding the crystalline graphite containing four graphene sheets. Both Brenner's reactive empirical bond order potential and Ito's interlayer intermolecular potential are adopted to represent `àBAB" stacking of graphite. The simulation results reveal that the mass of the incident species has a big influence on the absorption on and the reflection from the upside graphite surface, the peaks of which shift toward higher end side of incident energy as the mass increases. The absorption coefficient of the incident tritium is large, compared with that of the incident either hydrogen or deuterium. To penetrate the four- sheet graphite at some striking locations, deuterium and tritium need more kinetic energy. It is found that both the mass and the incident energy of the incident species affect the energy transfer to background substrate. These results would be important for understanding the tritium retention occurring in fusion devices.