A non-dispersive infrared (NDIR) gas sensor with an integrated optical gas chamber was designed to accurately detect multiple gas concentrations in a complex polluted environment. This chamber consists of a hexagonal prism shell and a reflection plate. In particular, six dual-channel infrared detectors for sensing different gases and one collimated infrared light source are integrated in the gas chamber. The IR light emitted by the collimated IR light source arrives at these detectors after four reflections. The result of simulation and data collection for optical path length and luminous flux shows that the average optical path length in the chamber is 63 mm, and the effective utilization rate of the luminous flux can reach 78.8%. The highly compact NDIR sensor can detect the concentrations of a mixture of gases (CO, CO2, CH4, H2CO, NH3, and NO with a volume fraction ranging from 0 to 4%).