Flue gas emissions of industrial and commercial boilers are the main pollution sources which are harmful to the atmospheric environment.To control the pollution and protect the environment

it is necessary to implement real-time and continuous monitoring on the flue gas composition

such as SO2 and NOx.Based on the differential optical Absorption spectroscopy（DOAS）

the system can real-time and continuously monitor the volume fraction of flue gas composition.A new single plug-in probe with four degrees of freedom is designed to make the optical path adjustment more convenient.It also improves the optical coupling efficiency.A spectrum self-adaptive algorithm is studied

which automatically adjust the integral time of the spectrometer according to the received spectral intensity.It improve the stability of the measurement.The relative measurement errors on the volume fraction of SO2 and NOx are less than ±2 %.