The invention provides a purifying device and method for gas in a closed space. The purifying device for the gas in the closed space comprises a container, a sensor, a first valve, a first purifying unit, a second valve, a second purifying unit and a gas circuit switch module, wherein target gas with the content being alpha is contained in the container, the content beta of the target gas in air is 0%-1%, and alpha is larger than beta; the sensor detects the content of the target gas in the closed space; the first valve is arranged on a gas way between the container and the first purifying unit; the output end of the first purifying unit communicates with the closed space; the input end of the second valve communicates with air, and the output end of the second valve communicates with the second purifying unit; the output end of the second purifying unit communicates with the closed space; and the input end of the gas circuit switch module communicates with the closed space. The purifying device has the beneficial effects of being small in gas consumption and the like.