The invention belongs to the technical field of optical detection and particularly relates to an ultramicro-gas concentration detection system. The system comprises a light source, a coupling optical path of the light source, an absorbing pond, an interferometer and an infrared detector. The absorbing pond comprises a tubular shell with the two ends sealed, and a reflection assembly arranged in the tubular shell. A right-angle mirror is arranged near the edge of a main mirror, when reflection light deviates from the main mirror, a reflection point on the main mirror side deviates by means of the right-angle mirror to form a new reflection path, and accordingly the reflection stroke is doubled. Thus, the shuttling stroke of infrared light beams is greatly increased, and detection requirements of ultramicro-gas are met.