In order to deal with the new features of complex AC/DC power grids, China is developing the construction for system protection. The broad control resources, complex control strategy and the great action response require real-time dispatching operation to fully grasp the operating state of the system and dynamically optimize the action strategy. This paper developed the overall frame of online monitoring and strategy optimization for large power grid system protection at first, including state monitoring, modeling and simulation, real-time analysis, strategy optimization, etc. Then, this paper introduced key technologies of the functions mentioned above. The specific method for strategy optimization is mainly discussed. The proposed online monitoring technology is applied to the frequency coordinating control system in East China Power Grid and the coordinating control system for the receiving end of Tianzhong UHVDC project in Central China Power Grid, which verified the effectiveness of the proposed method.