When faults and disturbances occur in power systems, backup power resources respond to maintain the system’s frequency, and the frequency recovery effect is related to the primary frequency regulation of the reserve capacity provided by and the response speeds of the system’s generating units and interruptible loads. The relationships between a system’s frequency and the frequency regulation capacity of the generating units and the interruptible loads are different. In this paper, an index for the frequency recovery effect is proposed based on the static frequency adjustment coefficient of the interruptible load and the adjustment coefficients of the generating units, and an optimization model of an interruptible load participating in primary frequency regulation is built with the objective of minimizing the cost of the primary frequency regulation reserve that uses the system’s frequency recovery effect as a constraint. The simulation results verify the validity of the model.