Abstract: In the deep geological repository for high-level radioactive waste, affected by the coupling of saline groundwater infiltration and nuclear decay heat, the buffer/backfill materials may undergo the chemical cycles of salinization and desalination process, which in turn affect their water retention capacity. The GMZ bentonite, the preferred buffer/backfill material in China, is investigated. Beishan in Gansu province, the potential disposal site, is taken as the engineering background, and the NaCl solution is chosen to simulate the chemical environment of in-situ groundwater. The water retention capacity tests considering the preliminary cyclical infiltrations of NaCl solution and distilled water are carried out. The results show that when the suction is below 150 MPa, the increase of the preliminary chemical cycles strengthens the water retention performance of the graphene-modified GMZ bentonite, and weakens the effects of dry density on the water retention capacity. When the suction exceeds 150 MPa, the preliminary chemical cycle scarcely plays a role. In the drying/wetting path, the water retention curve of the graphene-modified GMZ bentonite has obvious hysteresis, and the hysteresis amplitude gradually attenuates with the preliminary chemical cycles. When the preliminary chemical circles increase, the water entry and residual saturation value of the grapheme-modified bentonite increase, and the transition zone slope tends to be gentle. The influences of the chemical circles on the water retention capacity of the modified bentonite are related to the accumulation of salt content. With the increasing cycles, the salt content grows, and the influences of osmotic suction are more conspicuous.