A power system frequency control strategy based on the proportional-integral-derivative（PID） control method is designed. By detecting the variation of the circuit breaker status in an isolated power system， serious faults such as a generator outage and disconnection from the main grid can be identified， then the emergency pick-up step of the under-frequency load shedding（UFLS） device can be immediately started according to the capacity of the removed power， so the abrupt drop of frequency will be restrained. In the subsequent regulating process， the concept of PID closed-loop control strategy is proposed to trace and control the frequency change. Furthermore， the concept of the rough and fine-adjusting step is proposed to cope with the different phases of the frequency variation. The effectiveness of the control strategy proposed has been verified by EMTDC simulations. In addition， the PID frequency control algorithm can be extended to UFLS device of conventional power systems.