This paper presents a novel hybrid simulator， in which the digital computer and RTDS carry out the electromechanical and electromagnetic transient simulation， respectively. The overall design is proposed and the key techniques are studied in detail， including the interaction process between the two sides and the flow of synchronous simulation， the interface scheme for hybrid simulation， and the method for interface data exchange. A case study of the actual AC and DC hybrid power system containing the Gui-Guang HVDC system of China Southern Power Grid testifies to the practicability and validity of the overall design and the implementation of the hybrid simulator.