The invention discloses a constant pressure control system of a high pressure induced-draft fan. The constant pressure control system comprises a man-machine operation interface, a signal collection unit, a signal conversion unit, a signal processing unit and a frequency converter. The man-machine operation interface comprises an input keyboard and a displayer, wherein the input keyboard is used for inputting induced-draft pressure parameters. The signal collection unit is installed at an exhaust inlet of the high pressure induced-draft fan and used for collecting pressure signals of the high pressure induced-draft fan. The signal conversion unit converts the pressure signals into electric signals and transmits the electric signals to the signal processing unit. The signal processing unit conducts comparison between the electric signals and the induced-draft pressure parameters input into the man-machine operation interface to obtain a control signal, and then outputs the control signal to the frequency converter. The frequency converter regulates the rotating speed of a motor of the high pressure induced-draft fan according to the control signal. The constant pressure control system achieves the purposes that induced-draft pressure can be regulated according to corrugated boards with different gram weights, heat loss is reduced, and electric energy loss is lowered.