The invention discloses an automatic gain control (AGC) optimizing control method of a supercritical unit under coal quality changes, which comprises (1) correcting differential feed-forward of main vapor pressure deviation according to a coal quality correction coefficient British thermal unit (BTU), (2) correcting proportion feed-forward of unit load instructions according to the coal quality correction coefficient BTU, and (3) correcting proportion coefficient K of boiler master control according to the coal quality correction coefficient BTU, overlying the corrected differential feed-forward of the main vapor pressure deviation in the step (1) and the corrected proportion feed-forward of the unit load instructions in the step (2) to serve as new master control feed-forward of the unit boiler, enabling the corrected proportion coefficient K of the boiler master control in the step (3) to serve as an input quantity in a follow-up proportion integration differentiation (PID) step, and achieving output control of the unit boiler master control. The AGC optimizing control method of the supercritical unit under the coal quality changes has the advantages that by optimizing a logic structure of a unit boiler master controller, measures of sectional parameter varying are adopted specific to different coal quality, stability of main vapor pressure of the unit is effectively ensured, and control quality and key performance indexes of the AGC of the supercritical unit are improved.