A multi-terminal and closed-loop protective relays transient testing simulator， which locates at different substations， is proposed. Based on the electromagnetic transient calculation theory， the simulator realizes the precise synchronic simulation calculation by the synchronic calculation technique of the global positioning system（GPS） modules. The hardware and software of the simulation system and the calculation process of the real-time simulation are introduced. Signals of relay tripped are exchanged between different substations by the communication during the real-time simulation testing procedure. The testing for the transmission line relays at two substations is satisfied. It is adapt to testing for the transmission line protective relays with a channel in the power substations.