Voltage sag in power quality requires better timeliness. While most of the voltage sag analysis systems are based on off line mode, they all have a bad timeliness. Also voltage sag has propagation property in the power grid. Many monitoring sites will capture the voltage sag event, so in order to get better voltage sag timeliness, all data should be computed in different monitoring sites as soon as they are gathered. This distributed and real time analysis approach shares many similarities with multi-agent system. This paper proposes a JADE-based multi-agent technology to compute voltage sag events in distributed mode in real time. This technology has better timeliness because the analysis is proceeded earlier than the old centralized off-line analysis, which will help trace and resolve the abnormity in time and will definitely reduce the loss due to the abnormity.