The invention discloses a fully-integrated infrared gas sensor, which comprises a porous layer, an optical cavity layer, a circuit layer and a light-emitting diode, wherein a plurality of pores are formed in the porous layer, a plurality of minitype slots and infrared light source incidence windows are formed in the optical cavity layer, the light-emitting diode is arranged on the circuit layer, the porous layer and the light-emitting diode are connected so as to form an integrated gas chamber, and the infrared light source incidence windows are used for receiving infrared laser emitted from the light-emitting diode; the circuit layer is positioned on one side, back away from the porous layer, of the optical cavity layer, and is used for controlling the infrared laser, emitted by the light-emitting diode, according to the information of needed wavelength emitted by a characteristic infrared light source, and converting the infrared laser after being absorbed by the to-be-detected gas into an electrical signal, and the circuit layer is also used for calculating the converted electrical signal so as to obtain the concentration value of the to-be-detected gas. The invention further provides a working method of the infrared gas sensor.