Graphene oxide (GO) with extraordinary properties has been regarded as a good candidate to reinforce cementitious materials. Due to a variety of oxygen functionalities, GO can be well dispersed in aqueous solutions but unstable in the presence of alkaline cementitious solutions, which restricts the fully utilization of GO to the mechanical enhancement of cementitious materials. In this study, a stabilized GO aqueous solution by pre adding polycarboxylate-ether (PCE) copolymer was obtained before mixing with cement. The experimental results indicated that PCE can significantly improve the dispersion and stability of GO in alkaline cementitious solutions due to the stronger steric repulsion and hydrogen bonding. However, GO suddenly agglomerated when it was firstly mixed with cement by a rapid physical and chemical interaction, and post-addition of PCE has negligible helps to de-flocculate the GO. In terms of mechanical properties, cement paste with the stabilized GO-PCE showed higher flexural and compressive strength in comparison to that without PCE due to the better dispersion of GO in the hardened cement matrix. In conclusion, the prerequisite to make GO contribute more to the mechanical enhancement of cement paste is to ensure the good dispersion of GO in alkaline cementitious solutions achieved by pre-adding PCE.