To study the dynamic performance of the Southern China Power Grid in 2010， the system equivalence， model treatment， electromagnetic transient modeling method are presented for the multi-infeed AC/DC power system including±800 kV UHVDC. The PSCAD/EMTDC electromagnetic transient simulation software is used to establish the simulation model of 5 HVDC transmission lines， including±800 kV UHVDC from Yunnan to Guangdong. Detailed converter model is used in the study. The work has focused on the dynamic performance when various kinds of faults occur in the AC/DC system. Result presented in the paper provides the theoretical foundation for system planning and system security study in the Southern China Power Grid.