Flue gas emissions is one of major sources of environment pollution. The complex components and bad test environment bring great difficulties for the online detection. The continuous online monitoring system used for the multiple gas components analysis in flue gas which is based on FTIR spectrometer technique is introduced. In this system, the flue gas path is heated, and the multiple reflection cell is also thermostat heated. Those technologies are used to solve the problem of condensation and adsorption of flue gas in the process. The system can be used for the gas concentration monitoring of SO2 , HCl, oxynitride and the other gas in flue gas. A real-time comparison experiment between this system and CEMS was carried out with a good coincidence. The experiment shows that the online monitoring system based on FTIR spectrometer technique can be used in the measurement of concentration of multiple gas components in flue gas.