The effect of graphite oxide (GO) on the mechanical properties of oil well cement was experimentally studied, in view of the zonal isolation failure due to the brittleness. The microstructure of cement stone was observed by SEM, and the mechanism of graphite oxide reinforced cement stone was also investigated. The result illustrates that the mechanical properties of cement was improved significantly due to the addition of GO. When the GO dosage was 0.05%, the compressive strength, flexural toughness, and splitting tensile strength of cement paste at 7 day age were increased by 61.32%, 15.46% and 145.34% respectively. GO had no bad effect on the application properties of cement slurry, and the stability of the slurry was favorable. Besides, GO could reduce the fluid loss of cement slurry. When GO reinforced cement stone undergoes damage under complex stress, if damage occurs within graphite oxide layer, chemical bonds of GO layers must have been broken, and the force between the layers of GO must be overcome when interlaminar peeling happens, which will lead large amounts of energy consumption as well. The mechanical properties of the cement stone were enhanced by the above two aspects. The research results can provide a theoretical reference for solving the brittle fracture of oil well cement stone.