The invention provides a performance detecting method and system for a load control system. A light splitter, a tester and a host computer are added without changing a conventional structure of the load control system so as to respectively test action times of load shedding messages of a central station host, a man station host and a substation host and an action message exit time of a load control device, thereby determining whether the performance of the load control system meets requirements. Therefore, the stable operation of the system is ensured. The method of the invention has the advantages of simple operation, small error, low input cost and good reusability, can detect action message forwarding time of devices in the load control system in real time and on line, and can detect awhole group action time of the load control system in real time and on line.