A research has been conducted on the stability problem of time-delay power systems based on load frequency control (LFC) by establishing stability conditions on the basis of improved quadratic integral inequality and quadratic polynomial inequality. Firstly, a mathematical model is constructed of a load frequency delay system with PI parameters. Then, by introducing an augmented Lyapunov-Krasovskii functional and utilizing the sufficient conditions of the improved quadratic integral inequality and quadratic polynomial inequality in recent years, a stability criterion for systems with time-varying delays can be successfully derived. Finally, through the simulation of a load frequency system model, it is verified that the obtained stability criterion is superior in performance to some existing calculation results.