The addition of graphene oxide (GO) to cement paste significantly increases its toughness, however its fluidity is adversely affected. GO was first reacted with vinyltrimethoxysilane to produce a silane-modified graphene oxide (S-GO), which was then copolymerized with acrylic acid and a water reducing agent, isobutaenol polyoxyethylene ether (molecular mass 2400), to obtain a silane and copolymer-modified GO (P-S-GO) which acted as a water-reducing agent of the cement paste. A copolymer without GO was prepared under the same conditions for comparison. The structure, elemental composition and dispersibility of GO, S-GO and P-S-GO in a simulated cement environment (saturated lime water) were investigated by FTIR, XRD and a sedimentation test. The fluidity and rheology of the cement pastes with added GO, copolymer or P-S-GO were investigated. Results showed that vinyltrimethoxysilane reacted with the -OH group on the GO sheets to form an ether bond and the copolymer was grafted onto a vinyl group of S-GO by an addition reaction to produce the P-S-GO. The P-S-GO has a better dispersibility in saturated lime water with a negligible amount of aggregation compared to GO and a significant agglomeration occurs for GO. The addition of the P-S-GO to the cement pastes improves the fluidity and rheological properties compared with GO, offsetting the negative impact of GO and increasing the toughness of the resulting cements.