The simulation model of a±500kV/5000MW four-terminal direct current transmission system is proposed based on the real time digital simulator（RTDS）.The multi-terminal DC（MTDC）control and protection equipment is developed to form a real time closed loop simulation environment with RTDS.The control strategy employed for the system is the MTDC current margin method.The structure of control system is configured according to the MTDC master control and the converter station pole control.The MTDC master control coordinates the control of terminals with some functions such as power orders distribution，power ramping coordination，DC voltage control and MTDC communication.The converter station pole control contains conventional DC control functions and mainly realizes the power control in each station.The steady state operation， power compensation after single station blocking and DC line fault of the parallel four-terminal HVDC system are simulated using this simulation system.The simulation results show that the proposed control strategy and the developed MTDC control protection equipment can meet the requirement of the MTDC power transmission.