After the asynchronous interconnetcion of Yunnan Power Grid (YPG) and the main grid of China Southern Power Grid (CSG), the operation characteristics of power grids will have a great change, and the problem of frequency instability will become one of the major risk of power system operation. Based on the real-time simulation platform of CSG AC & DC hybrid power system, an overall simulation system of YPG and CSG in asynchronous interconnection operation is constructed, and a series of simulations on the frequency responses of YPG and control characteristics of HVDC projects in asynchronous interconnection operation are carried out considering the frequency control measures of primary frequency modulation, frequency limitation controllor of HVDC and special protection system. In comparison between simulation results and site records, the governor and turbine models are optimized to improve the simulation accuracy of the control characteristics of CSG in asynchronous interconnection operation, which provides technological support for the asynchronous interconnetcion project of YPG and CSG.