Abstract: The frequency control function of UHVDC transmission system can use the rapid control ability of DC system transmission power to adjust the active power balance of AC power grid at both ends to improve the performance of AC system.The principle of frequency control function of UHVDC transmission system was introduced, the action characteristics of frequency control function of UHV rectifier converter station were simulated, and the measures to shield the frequency control function were presented in case of commutation failure, so as to provide reference for the function optimization of UHVDC frequency controller.