An AC-DC real-time simulation system is developed， in which an equivalent model of China's southern power grid is built on a real-time digital simulator（RTDS）， while the external actual DC control and protection devices are connected in a closed loop. The key problems in modeling are discussed and suggestions are made to improve the RTDS model. Two typical actual faults have been successfully simulated by use of the RTDS model. The real causes of faults are analyzed， and some improvement measures are put forward. The simulation results are compared against the field fault recordings. It is demonstrated that the simulation models proposed are accurate and reliable. The RTDS model established can accurately reflect the dynamic characteristics of the real power system.