An online temperature monitoring system for flue gas based on acoustic pyrometry was developed and installed in a boiler of 200 MW. Hot-state experimental research of acoustic source signals included linear sweeping frequency signal and pseudo random binary sequence were tested, and the temperature was measured by K ceramic thermocouples which were specially made for verification. Research indicates that those signals can be used as sound source signals due to the treatment. The error of temperature in the measurement used acoustic pyrometry which is stable and reliable can be less than 1%, and it provides important consultation for this technology popularizing application.