The invention discloses a desulfurization wastewater zero discharge system for gradient utilization of flue gas waste heat and a working method of the desulfurization wastewater zero discharge system,and belongs to the technical field of desulfurization wastewater treatment. The system comprises a first air inlet device, a second air inlet device, a first air heat exchanger, a second air heat exchanger, a first flue gas heat exchanger, a second flue gas heat exchanger, a spray drying tower, a dust remover, an evaporation concentration tower and a desulfurization wastewater pool. High-temperature flue gas at the tail of a boiler is used for increasing the air temperature, desulfurization wastewater is evaporated and dried by high-temperature air through gradient utilization of the temperature, the cooled air is used for concentrating and reducing the desulfurization wastewater, the dust remover is used for recovering dried salt in the wastewater, and the air heat exchanger is used forrecovering heat in air at an outlet of a system for preheating the air. Compared with a conventional desulfurization waste water zero-discharge method, waste heat in flue gas is fully utilized, concentration and drying of waste water are synchronously achieved, the influence on water balance and material balance of gas in an original flue is small, and good application prospects are achieved.