The invention discloses an online monitoring method for the
flue gas temperature of a
hearth outlet of a power boiler, comprising the steps of: placing a plurality of targets at a vertical section near a smokestack of a boiler for sensing the temperature; installing a
near infrared imaging device on the side wall of a horizontal
flue of the boiler for collecting the
radiation energy of the targets; carrying out communication by the
near infrared imaging device with a computer through a
serial port and RS232, sending the collected
radiation energy of the targets to the computer, calculating by the computer to obtain temperature data of the targets; and then sending the temperature data of the targets to a host computer for
processing, and converting the temperature to the
flue gas temperature of the position of the targets. The invention has the advantages that the average temperature at the section of the
hearth outlet, the average temperature of the upper / lower and left / right side, and the highest / the lowest temperature can be continuously monitored, the distribution curve of the temperature of the section can be drawn, and a plurality of alarm functions are set; and the invention is beneficial to the safe and economic operation of a boil with a
large capacity and high parameters.