A CO₂ detection system based on a broadband light source and a Fabry-Pérot resonant cavity is presented. By combining with other active optical system error analysis methods, the error terms of the system are quantitatively analyzed. The main error source of the system includes four parts: the energy fluctuation of the light source, the crosstalk of the circulator, the change of the FP cavity spectral line and the noise of the detector. By modeling and calculating the influences of independent error items on the difference signal value, four main systematic errors are determined. In the error analysis of traditional atmospheric detection, the hardware system error is related directly to the concentration data. Instead, in the error analysis proposed, the hardware error and the inversion algorithm error are stripped. This method is of better guiding significance to the improvement of system operation ability.