When the power is transmitted from large hydropower station or thermal power plant located far away from the main power grids in HVDC project, the AC power system at sending end forms an island system. In Yunnan-Guangdong UHVDC transmission project, the DC frequency control function in island operation mode is simulated and analyzed by real-time digital simulator and the control&protection system of SIMATIC TDC. In addition, the shortcomings of the frequency control functions found in the simulation are analyzed, and the practicable improving measures are given. It is proved by the simulation that the improved measures are effective.