The invention discloses an airtightness test and nitrogen replacement device used for a high-pressure air cylinder, which belongs to the technical field of the airtightness test. The airtightness test and nitrogen replacement device used for a high-pressure air cylinder comprises a liquid nitrogen storage tank, a self-boosting gasifier, a liquid nitrogen pump, a water bath type gasifier, an air temperature type gasifier, an energy storage device, an airtightness test operation and control system and a frame, wherein the liquid nitrogen storage tank and the self-boosting gasifier are horizontally arranged in the frame side by side; the front end of the frame is provided with the liquid nitrogen pump, the air temperature type gasifier and the water bath type gasifier; in the frame, one end of the liquid nitrogen storage tank is connected with the self-boosting gasifier, the other end of the liquid nitrogen storage tank is connected with the liquid nitrogen pump, the liquid nitrogen pump is connected with the water bath type gasifier, and the water bath type gasifier is connected with the air temperature type gasifier; outside the frame, the air temperature type gasifier is connected with the energy storage device and the energy storage device is connected with the airtightness test operation and control system. The airtightness test and nitrogen replacement device has the advantage that the problems of high operation cost and maintenance cost, unstable working state, high incidence rate of various faults, complex production work, low production efficiency and the like can be solved.