Abstract: The time delay problem caused by the dynamic frequency modulation with the communication network in the power system will pose a great threat to the system stability. Aiming at the network time delay problem in the load frequency control, a discrete state space model of the load frequency control considering the time-varying time delay characteristics is established. Using multiple summation inequalities, a low-conservative Lyapunov stability criterion is constructed to solve the stability margin of the interval time-varying delay LFC system. On this basis, the LMI criterion of the delay-related output feedback gain is derived to design the parameters of the secondary frequency modulation controller. The simulation results show that the stability criterion proposed in this paper can solve a larger time-delay stability margin with less conservative. The proposed controller design criterion can quickly solve the output feedback gain under time-varying time delay, effectively suppress the frequency oscillation of the system under small disturbance, giving the system time delay stability.