The invention provides a detection device for a weak absorbing gas in a complex environment. The detection device comprises a first light source, a second light source, a first detector, a second detector, and an analysis module, wherein the wave length of first measuring light emitted by the first light source covers the absorption line of the weak absorbing gas; the wave length of second measuring light emitted by the second light source covers the absorption line of a strong interfering gas in the complex environment; in the absorption area of the weak absorbing gas of the first measuring light, the absorption of the strong interfering gas is 10 times larger than that of the weak absorbing gas; the first detector is used for converting the first measuring light penetrating the complex environment into a first electric signal, and sending the first electric signal to the analysis module; the second detector is used for converting the second measuring light penetrating the complex environment into a second electric signal, and sending the second electric signal to the analysis module; the analysis module is used for processing the received second electric signal according to the optical absorption spectrometry, so as to obtain the content of the strong interfering gas, and obtain the type and content of the weak absorbing gas according to the differential optical absorption spectrometry and the content of the strong interfering gas. The detection device has the advantages that the accuracy is high; the structure is simple; the cost is low.