The invention discloses a system and a method for treating desulfurization wastewater based on concentration and bypass flash evaporation. The system comprises a desulfurization tower, a wastewater pretreating and concentrating device, a wastewater pump, a wastewater evaporator, high-temperature flue gas supply equipment and tail gas recycling equipment, wherein a liquid outlet in the bottom of the desulfurization tower is connected with the wastewater pretreating and concentrating device in a matching manner; a smoke outlet in the top of the desulfurization tower is connected with a chimney;a liquid outlet of the wastewater pretreating and concentrating device is connected with a liquid inlet in one side of the wastewater evaporator through the wastewater pump in a matching manner; a gasinlet in the top of the wastewater evaporator is connected with the high-temperature flue gas supply equipment in a matching manner; a gas outlet in one side of the wastewater evaporator is connectedwith the tail gas recycling equipment; the high-temperature flue gas supply equipment comprises a boiler, a fan and a coal economizer; a smoke outlet of the boiler is connected with a gas inlet of the coal economizer in a matching manner. The system has the characteristics of large wastewater treatment amount, high flue gas utilization rate, no influence from the load of the boiler, and low investment and operating cost.