The electromagnetic transient modeling method was presented for the system, the system equivalent and model treatment were also concerned. The EMTDC (electromagnetic transient simulation software) was used to establish the simulation model covered a total of 5 HVDC transmission lines, including a ±800kV UHVDC transmission system from Yunnan to Guangdong. The use of detailed converter model can solve the problem of limitations in previous studies. The research focused on the dynamic stability and commutation failure when kinds of faults occurred in AC/DC system. The result of the work has provided a theoretical foundation for such system, also provided a feasible way for the dynamics study of large-scale multi-infeed HVDC system by using of the electromagnetic transient simulation model.