The frequency emergency coordination control system plays an important role in dealing with the frequency stability problem of power systems caused by multi-infeed HVDC systems.However,the rapidity and accuracy of the coordinated control system largely depends on the DC coordinated control function.Firstly this paper introduced the interface between DC control system and coordinated control system,and interface data processing method was appropriately designed.Then a new DC power fast decreasing control method was proposed to solve the problem that the coordinated control system cannot distinguish the decrease of DC power under the current control method.A modulation power compensation method and a commutation failure delay modulation method were also proposed to improve the accuracy and reliability during the implementation of modulation control.Finally,the correctness of the functions was verified by RTDS simulation test,which provides reference for the application of the UHVDC coordinated control function.