The invention relates to a treating apparatus for waste liquid produced in sodium-process flue gas desulfurization. The treating apparatus mainly comprises a flue gas/desulfurization waste liquid heatexchanger, a dust-removal and desulfurization tower, a solid-liquid separator, a plate heat exchanger and a steam compressor, wherein the flue gas/desulfurization waste liquid heat exchanger is divided into an upper heat exchange zone and a lower crystallization zone; the upper heat exchange zone comprises a steam chamber, an upper tube plate, a partition plate, a heat exchange tube and a lower tube plate from top to bottom; a desulfurization waste liquid feeding box is arranged between the upper tube plate and the partition plate; a liquid film former is arranged at the heat exchanger tube end of the desulfurization waste liquid feeding box; the lower crystallization zone is provided with an inclined drainage plate, a sealing plate and a crystallization tank; and the sealing plate is inserted into a position located below the liquid level of the crystallization tank. Through adoption of the specific treating apparatus and a specific treating method, the dual purposes of allowing purified flue gas to meet emission requirements and realizing zero discharge of waste liquid produced in flue gas desulfurization are achieved on the basis of full use of the waste heat of flue gas; and in particular, the supplementation amount of fresh water for a flue gas desulfurization system is reduced, and overall treatment cost of flue gas desulfurization and desulfurization waste liquid is lowered.