A Perturbation Observer based Sliding-Mode Control (POSMC) is proposed to enhance the stability of multi-machine power systems. First, the nonlinearities, parameter uncertainties, unmodelled dynamics and time-varying external disturbances of the system are aggregated into a perturbation, which is estimated by a Sliding-Mode State and Perturbation Observer (SMSPO) in real-time. Then, the estimated perturbation is fully compensated by the sliding-mode controller to achieve a globally consistent control performance. POSMC has the advantages of simple structure, high reliability and easy implementation, while it does not require an accurate system model and only one state, e.g., rotor angle, measurement is needed. Finally, three case studies, e.g., mechanical power variations, three-phase short-circuit fault and generator parameter uncertainties, are undertaken, which verify the effectiveness and robustness of POSMC, i.e., it can achieve the best dynamic performance under various operation conditions, and can effectively restore the power system after failure. This work is supported by National Natural Science Foundation of China (No. 61963020).