The chloride penetration resistance of cement mortar modified by graphene oxide (GO) was studied by rapid electric flux method and long-term chloride salt immersion test. The relationship between electric flux and apparent chloride diffusion coefficient was analyzed. The microstructure of the mortar was tested by scanning electron microscope (SEM). The results show that the incorporation of appropriate amount of GO improves the chloride penetration resistance of cement mortar. When the content is 0.06% (mass fraction), it is the most significant. Compared with the control group without GO, the electric flux and chloride erosion depth of cement mortar are reduced by 34.5% and 27.2%, respectively. After long-term chlorine salt immersion for 60 d and 120 d, the free chloride concentrations at 0~5 mm decrease by 28.4% and 15.3%, respectively. There is a good linear relationship between electric flux and apparent chloride diffusion coefficient. GO effectively regulates the shape of hydration products, reduces internal pores, and makes its microstructure more dense, thereby improving the chloride penetration resistance of cement mortar.