The aim of the invention is to provide a device and a method for joint evaluation of denitration and demercuration performances of a catalyst. The device comprises pipelines I, II, III, IV, V and VII, a pressure reducing valve, a filter, a mass flowmeter, a one-way valve, a preheating mixer, an electric heating furnace, three-way ball valves I, II and III, a flue gas analyzer, a mercury analyzer, and a catalytic reactor with two gas inlet and a gas outlet. Actual conditions of a factory are simulated by production of flue gases with different ratios. The catalyst with good effects both on denitration and demercuration specific to a specific flue gas can be selected by replacing of different types of catalysts.