This paper investigates the theoretical saturation sulfur capacity of a renewable wet flue gas desulfurizer, sodium phosphate buffer solution. The saturated pH value and desulfurization capacity in the sodium phosphate buffer solution were calculated by means of ion balance, dissolution equilibrium and numerical calculation software. The results were tested and verified by experiments. The results show that the change law and the result are consistent. The sulfur capacity increases with the increase of the concentration of phosphate in the buffer solution, which indicates that the theoretical calculation method is reasonable and feasible. The results can provide basic data and reference for phosphate flue gas regeneration desulfurization processes, such as the liquid-gas ratio, solution pH value, and phosphoric acid concentration.