The invention provides a non-chromatic-dispersion infrared gas sensor capable of lowering detection lower limit. The sensor comprises an infrared transmitter, a first transmission window, an air chamber, a second transmission window, an infrared sensor and a control module, and the first transmission window and the second transmission window are arranged to be identical or different planar lenses, convex lenses, concave lenses or Fresnel lenses; the infrared detector comprises at least one reference channel and at least one measurement channel group, each measurement channel group comprises at least two measurement channels for detecting same predetermined gas, a plurality of light filters are arranged between the reference channels as well as the measurement channels and the second transmission window, and the light filters adopted by each measurement channel in a same measurement channel group are uniform in specification. By adopting the non-chromatic-dispersion infrared gas sensor and a detection method, collection amount of current signals or voltage signals within unit time is increased effectively, gas concentration detection lower limit is lowered, and resolution is improved.