The present invention relates to a feedforward compensation design method of a thermal power generating unit AGC system, and relates to the field of thermal power generating unit AGC system control strategy. The objective of the invention is to solve the problem that a current AGC performance optimization method is difficult to improve three examination index requirements of the AGC at the same time. The method comprises the steps of: determining whether an AGC mode is input to the unit or not, if the AGC is not input into operate, allowing power signals PT transmitted to a steam turbine regulator to be still actual power signals of the unit, ending the method, and if the AGC is input into operate, executing a step 2, namely performing feedforward compensation design optimization of the power signals transmitted to the steam turbine regulator; the step 2: calculating power deviation [Delta]P0:[Delta]P0=P'-P according the current load P' and an actual load P of the AGC, calculating constants [Delta]P1, [Delta]P2:[Delta]P1=W*K1 and [Delta]P2=W*K2 by employing a unit rated load W; and the step 3: determining a size relation of a power instruction deviation [Delta]P0 and the constant [Delta]P1, and calculating corrected power signals PT by employing the constant [Delta]P2 according to a determined result, and transmitting the corrected power signals PT to the steam turbine regulator. The feedforward compensation design method of the thermal power generating unit AGC system is suitable for a thermal power generating unit AGC system.