The invention discloses a method for achieving optimizing control of the load of a thermal power generating unit in a PROPR mode. The method specifically comprises: step one, an AGC load instruction value issued by a dispatching center is analyzed and optimized through a function; step two, the corrected AGC load instruction value and a unit actual power value are subjected to different and judgment to generate the steam turbine master control feed-forward amount; step three, the corrected AGC load instruction value and the unit actual power value are subjected to the different and judgment to perform load change rate switching. The method performs feed-forward optimizing adjustment on a steam turbine master control of the unit by analyzing the load instruction value and the actual power value of the unit and judging the load real-time change situation of the unit, performs broken line optimizing adjustment on the load change rate of the unit, accordingly improves the adjusting accuracy and a response time index effectively while adjusting the rate index of the unit in the PROPR mode, and ensures supply and demand balance of power grid load and the stability of power grid frequency.