The invention relates to a self-testable SCR denitration catalyst activity detecting device and a self-test method. The self-testable SCR denitration catalyst activity detecting device comprises a plurality of air supply pipelines, a water injection pump, a water storage tank, a preheating furnace, a reaction furnace and a calibration gas pipeline, a plurality of air supply pipelines are both provided with mass flowmeters and valves, a plurality of air supply pipelines are connected in parallel to form a mixed gas pipeline and connected with an inlet of the preheating furnace, the mixed gas pipeline is provided with a mixed gas valve, the inlet of the water injection pump is connected with the water storage tank, an outlet of the water injection pump is connected with the inlet of a preheating furnace, the outlet of the preheating furnace is connected with the inlet of the reaction furnace, test sample is arranged in the reaction furnace, the calibration gas pipeline is connected withthe mixed gas pipeline and located in the former of a mixed gas valve, the calibration gas pipeline is further provided with a mass flowmeter and a valve. The detecting device can calibrate gas flow of each pipeline and rate of water adding, and synchronously complete the detection of gas tightness of a device and stability of flow field of a gas temperature field in a reaction unit.