The present invention provides a method and a system for optimizing primary frequency modulation (FM), and a terminal device. The method for optimizing the primary FM comprises: obtaining a unit system of the primary frequency failure period and an operating state of the unit system; obtaining the unit speed, and the measured parameters of the measured model of the primary FM, wherein the measuredparameters comprise the characteristic parameter value of the steam turbine model, the PI controller parameter value, the rotational speed difference, the load delay parameter value, the FM instruction delay value, the PI controller output rate limit value, and the pressure pullback loop parameter value; inputting the different values of the measured parameters in the preset range and the unit speed to the measured model of the primary FM to obtain the simulation result; and optimizing the primary FM according to the primary FM performance reflection function and the simulation result. According to the technical scheme of the present invention, by using the simulation means to determine the dominant factors and optimize the control strategy, the blindness of the conventional method is avoided, the labor intensity and the test risk are reduced, the coupling influence can be solved, the key parameters can be determined, and the pertinence of the primary FM analysis optimization can be improved.