NDIR infrared gas sensor

The utility model discloses an NDIR infrared gas sensor. The NDIR infrared gas sensor comprises a gas cavity upper cover and a circuit board, the air cavity upper cover and the circuit board are matched to form a closed light path air cavity; an air inlet hole is formed in the air cavity upper cover; a light source reflecting surface, a first-stage reflecting surface, a second-stage reflecting surface, a third-stage reflecting surface and a receiver reflecting surface are arranged on the inner side of the air cavity upper cover, the light source reflecting surface, the second-stage reflecting surface and the receiver reflecting surface are located on the same side of the light path air cavity, and the first-stage reflecting surface and the third-stage reflecting surface are located on the other side of the light path air cavity; the circuit board is provided with a light source and a receiver, and the light source and the receiver are arranged at two adjacent corners of the circuit board and are respectively