Effect of cement mixture containing bagasse ash (SCBA) and slag (BFS) on the corrosion properties of steel in sea-sand mortar was investigated. The electrochemical impedance spectra, open circuit potential, and corrosion current density of steel were determined, and the chloride ions content, pH value and hydration products were measured. The results show that an increase in bagasse ash content leads to an increase in the corrosion rate of steel and [Cl-]/[OH-] value. However, an increase in slag content can reduce the corrosion rate of steel, and cause that the [Cl-]/[OH-] value initially increases and slightly decreases after 28 d. The addition of 15% bagasse ash and 15% slag has a positive synergistic effect, effectively increasing the resistance of mortar protective layer and charge transfer resistance, improving the stability of the passivation film, and reducing the corrosion rate of steel in the mortar by 31.86%, compared to the reference group. The optimum corrosion resistance of the steel can be obtained. It is indicated that the use of a cement mixture containing bagasse ash and slag can effectively improve the corrosion resistance of steel in sea-sand mortar.