With the increase of new energy power generation, the frequency fluctuation of the power grid intensifies. To accelerate the elimination of power grid frequency deviation, the automatic generation control (AGC) command optimal dispatching method was proposed based on the relationship between power output changes and maximum ramp rates of units. Firstly, data segments with continuous increasing and decreasing trends were obtained from historical data of units by the piecewise linear representation method. The amplitude change of each data segment and the corresponding ramp rate was calculated. The relationship between the unit output changes and the maximum ramp rates was established by polynomial fitting based on appropriate samples of amplitude change and ramp rates. Then, considering the constraints of output range and ramp rates under different output changes of units, the AGC demands of power generation units were determined by minimizing the AGC demand response time of the power grid. Finally, numerical and industrial examples including a comparison with existing dispatching methods were provided to illustrate the effectiveness of the proposed method.