The double alkali method is a kind of flue gas purification technology using sodium based desulfurizer for desulfurization in the tower. Due to the strong alkalinity of sodium based desulfurizer, the solubility of reaction products after absorption of sulfur dioxide is large, which will not cause supersaturation crystallization and scale blocking. The process is to use circulating alkali solution to spray the flue gas in reverse direction in the absorption tower to wash the sulfur dioxide in the flue gas. The desulfurized products after washing are discharged into the regeneration tank, and then the regenerated sodium based desulfurizer is recycled in the desulfurizing tower. Double alkali desulfurization process has less investment and operation cost, so it is not easy to cause supersaturated crystallization and scale blocking. It is suitable for desulfurization transformation of small and medium-sized boilers. The process flow is as follows: