Abstract: The paper introduced design method of structure of optical air chamber of infrared methane sensor in details, and derived that the optimal light path length of the optical air chamber is 60 mm, and the tests show that the sensor responses faster when diameter of the air chamber is 10~30 mm, thus determined the diameter of the infrared air chamber as 20 mm. It used RCS2000(A) automatic gas distributing system to make experimental test for the infrared methane sensor based on the optical air chamber, the test results validate good feasibility and sensitivity of the gas chamber.