The invention discloses a gas detection method and system. The gas detection system comprises the following parts that an optical path unit is used for generating two paths of same optical signals; agas chamber unit is divided into a reference gas chamber and a measurement gas chamber which are isolated from each other, wherein reference gas with preset concentration is sealed in the reference gas chamber, and the measurement gas chamber is communicated with the environment where external gas to be measured is located; the two paths of same optical signals respectively enter the reference gaschamber and the measurement gas chamber, the two paths of optical signals are attenuated to different extents after being absorbed by the reference gas chamber and the measurement gas chamber, and the reference gas is the same as the external gas to be measured in type; and a detection unit is used for detecting the two paths of optical signals absorbed by the reference gas chamber and the measurement gas chamber, and determining the concentration of the external gas to be measured according to the intensity of the two paths of optical signals and the concentration of the reference gas. Measurement errors caused by light source optical power jitter and external environment noise are reduced, and the detection precision and sensitivity of the system are improved.