The invention discloses a power and communication semi-physical simulation system based on RTDS and SDH, and a construction method, and belongs to the technical field of power system simulation. The system comprises a power semi-physical simulation environment and a communication semi-physical simulation environment. The power semi-physical simulation environment consists of an RTDS real-time power simulation end and power secondary equipment. The communication semi-physical simulation environment consists of an SDH physical network and a communication simulation end, and the SDH physical network comprises SDH equipment, an optical fiber, and related accessory equipment. The RTDS real-time power simulation end is connected with the power secondary equipment. The communication simulation end is connected with the SDH physical network and the power secondary equipment through a protocol converter, and the power secondary equipment is connected with the SDH physical network. The time correction and synchronization of the RTDS real-time power simulation end, the power secondary equipment and the communication simulation end are carried out through a GPS clock. The system can effectively simulate the strong-coupling power and communication system, and provides an effective verification testing platform for the power secondary equipment and a corresponding communication interface.