Objective To study the effects of the composite corrosion inhibitor composed by odium erythorbate, sodium hexametaphosphate, D-sodium gluconate and zinc sulfate on the corrosion rate of carbon steel under the condition of softened water. Methods The effects of the composite corrosion inhibitor on the corrosion rate of carbon steel were studied under the conditions of different dosages, pH values and temperature using the static specimen method and rotation specimen method. Results Under the conditions of softened water pH=10, corrosion inhibitor dose of 150 mg/ L and temperature of 50 ℃, the composite corrosion inhibitor showed good corrosion inhibition effect on carbon steel, and the corrosion rates and inhibition ratios were 0. 0303 mm/ a, 86. 31% and 0. 0350 mm/ a, 96. 12 % for carbon steel in the experiments using the static specimen method and the rotation specimen method, respectively. Conclusion The corrosion rate of carbon steel in softened water decreased with the increasing dosages of composite corrosion inhibitor and the rising pH value, and increased with the increasing temperature.