When the groundwater contains a high concentration of carbonate and chloride ions, the corrosion of carbonic acid solution on underground concrete structures will lead to a faster migration of chloride ions in concrete. Migration characteristic of chloride ions in mortar exists in carbonic acid solution environment. The corrosion products and microstructure of paste under the attack of carbonic acid solution and chloride ions were analyzed by X-ray diffraction (XRD), thermogravimetric analysis (TG-DTG) and scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS). The results show that in the early stage of carbonic acid water corrosion, calcium hydroxide and C-S-H gel decalcifies to form calcium carbonate filling the pore structure of the mortar, and inhibits the migration of chloride ions in the mortar. At the later stage of corrosion, the dissolution of calcium carbonate and the conversion of C-S-H gel with a low calcium-silicon ratio into amorphous silica gel lead to the increase of mortar porosity and the acceleration of chloride ion migration in mortar. Carbonic acid water corrosion results in the decomposition of Friedel's salts and the decrease of the adsorption capacity of C-S-H gel for chloride ions. Reducing water-cement ratio can increase the corrosion resistance of mortar to carbonic acid water and reduce the migration rate of chloride ions.