In order to prevent long term voltage instability induced cascading events; a multi agent system (MAS) based wide area protection strategy is proposed in this paper. Due to some unexpected failures of control strategy execution or the consequent unexpected disturbance, the power system will face more complex emergent situation than planned. The process control strategy will be applied to improve the effectiveness and reliability of MAS based control strategy. The supervisory agent will help control center in the high level to manage not only the regular control process against the estimated emergent states, but also those unusual control process variations when unexpected situation is experienced. A hybrid simulation platform based on MATLAB/LabVIEW and real time digital simulator (RTDS) is set up to simulate a voltage collapse case in the power system of Eastern Denmark and demonstrate the effectiveness of the improved MAS based protection strategy.