In this paper, a scheme for on-line monitoring of multi-component decomposition based on TDLAS is presented, which is mainly composed of hydrogen sulfide, carbon monoxide and hydrogen fluoride. Considering that the most important problem of multi-component gas detection is the crossover of components, the characteristic spectra were first analyzed. The characteristic absorption peaks of hydrogen fluoride are very strong, which makes it easy to detect hydrogen fluoride with high sensitivity. However, the absorption peak of hydrogen fluoride covers hydrogen sulfide and carbon monoxide, making it difficult to detect. The interference problem is analyzed and the solution is given. In addition, multi-parameter transmitter in power industry is used to provide data for on-line calibration. The concentration of sulfur hexafluoride is retrieved from SF6 density information provided by multi-parameter transmitter. According to the temperature information, decomposition gas concentration value is calibrated. In this paper, an on-line monitoring device for SF6 decomposition mixture gas based on laser absorption spectroscopy is developed. The results of temperature cycle test show that the influence of environmental parameter changes on the concentration results can be reduced after on-line calibration.