The invention discloses a primary frequency modulation control system and method applied to a thermal power generating unit and based on safe and stable operation. The system comprises a first subtractor module and a high-low limit alarm module, wherein a unit rate limited afterload instruction value and a load measured value are accessed to an input end of the first substractor module respectively, and a difference between the unit rate limited afterload instruction value and the load measured value is sent to the high-low limit alarm module; a high limit exceeding alarm signal and a low limit exceeding alarm signal of the high-low limit alarm module are accessed to set ends of a first switch selector and a second switch selector respectively; the first switch selector and the second switch selector are connected to a block increase end and a block decrease end of a turbine PID controller respectively. According to the system and the method, the priority of primary frequency modulation actions during grid frequency fluctuation can be effectively guaranteed, coordinated operation of AGC (automatic gain control) and primary frequency modulation of the unit can be improved, the frequency fluctuation of a grid system is reduced, and accordingly, safe and effective operation of electrical equipment and power equipment of users is guaranteed.