The invention provides a micro far infrared type gas concentration monitoring method and device. According to the device, a dual wavelength infrared monitoring principle is adopted, a reasonable micro structure design is supplemented, a micro focus-free multiple reflection gas chamber is formed; the inner wall of the gas chamber is provided with a reflection film in a plating manner, so that the reflection on infrared light can be improved, enough information can be obtained by an infrared detector to reflect the concentration of monitored gas in a tiny space; a diaphragm and a window film are additionally arranged to converge a far infrared light source, so as to obtain far infrared light needed by measurement; the structure of a filtering net and a waterproof permeable film are adopted for performing filtering and waterproof protection, and the monitoring precision is guaranteed while the gas chamber and optical elements are also protected. The device is provided with data output pins and alarming output pins which can be used for outputting digital signals of the gas concentration and alarming signals of an open collector and is capable of meeting the demand of gas concentration alarming on hazardous occasions.