The harmless process for the sodium removal of red mud is investigated by using CO2 to remove sodium from the aluminum solid waste Bayer red mud. The indicator of the experiment is the sodium removal rate. Reaction temperature, reaction time, solid-liquid ratio, CO2 gas flow, and the particle size of red mud five impact factors for the sodium removal are researched and the results are as follows. The reasonable parameters for the sodium removal are reaction temperature 25 ℃, reaction time 100 min, solid-liquid ratio 1/5, CO2 gas flow 0.8 L/min, and fine grinding red mud particle size. Under the reasonable parameters condition, the sodium removal rate is 76.12%, the remaining Na2O content is 2.5%, and the concentration of NaHCO3 solution is about 4%. Compared with the traditional process, the process is simpler. By the process, the PH value of red mud slurry is decreased to 7.1 from 10.2, and the red mud becomes harmless. During the process, the emission of greenhouse gas CO2 is reduced and NaHCO3 can be recycled, conducive to recycling of resources.