Infrared gas chamber is an essential part of a non-dispersive infrared methane sensor and directly affects the performance of the sensor. In this study, based on the Lambert-Beer law, a reflective A-type gas chamber was designed, and the corresponding parameters of the gas chamber were optimized by simulation. Compared with the direct gas chamber, the optical-path length of the reflective A-type chamber is about twice that of the direct gas chamber with the same volume and outline. And compared with the existing reflective chambers, although there is no significant difference about the optical-path length-to-area ratio, the luminous flux utilisation efficiency of the reflective A-type chamber is increased by 57.827%. Overall, the new gas chamber designed in this study compensates for the short optical-path length of the traditional direct gas chamber and the low luminous flux utilisation efficiency of the existing reflective gas chamber, and has the advantages of long optical-path and high luminous flux utilisation efficiency.