The invention discloses a device and a method for eliminating NOx in flue gas, and the device and the method solve the problem that cost of an existing plasma denitration method is high. The device comprises a flue gas conveying pipeline, an ammonia storage tank, an electron beam generating device, a plasma reactor and an ammonia evaporator, wherein flue gas generated by a boiler is connected to the plasma reactor through the flue gas conveying pipeline; water vapor generated by the boiler is conveyed to the flue gas conveying pipeline; liquid ammonia in the ammonia storage tank generates ammonia gas through the ammonia evaporator and the ammonia gas is conveyed to the plasma reactor; and the electron beam generating device is connected to the plasma reactor. When flue gas passes through the ultrahigh-voltage plasma generator, water vapor in the flue gas is firstly ionized into -OH and -O, and NO in the flue gas is oxidized into NO2; and in the presence of ammonia gas, NH4NO2 is generated and is easily decomposed into N2 and H2O, so that the denitration purpose is reached. The device is simple, is high in denitration efficiency, and is low in cost of water vapor and ammonia gas.