The invention relates to a system and a method for evaporating desulfurization wastewater by using exhaust gas waste heat under an underload working condition. The system comprises an economizer fumebypass subsystem, a desulfurization wastewater high-temperature flue bypass evaporation subsystem as well as an air pre-heater bypass flue, wherein the economizer fume bypass subsystem comprises an economizer bypass flue; the desulfurization wastewater high-temperature flue bypass evaporation subsystem comprises a softening tank, a concentration device, a wastewater pump, a wastewater preheater and a double-fluid atomization ejector which are connected in sequence through a wastewater pipeline; the softening tank is connected with a desulfurizer; an evaporator is arranged on the air pre-heaterbypass flue; the double-fluid atomization ejector is arranged at the top end inside the evaporator. The invention further relates to a method for achieving zero desulfurization wastewater emission under the underload working condition on the basis of the system for evaporating the desulfurization wastewater by utilizing the exhaust gas waste heat. By adopting the system and the method, the energyutilization rate of a machine set is increased, and zero wastewater emission is achieved.