Abstract: In order to ensure the security and stability operation of ultra-high voltage（UHV） AC/DC power grid in China, State Grid Corporation of China actively constructs the system protection of power grid to improve the comprehensive security prevention and control capability. However, with the application of cross-region DC as a flexible control resource in the system protection, it must be considered that the rapid change of DC power will affect the sending-end and receiving-end power grids simultaneously. On the basis of sorting out the system protection construction of power grid in China, the impact of frequency response based DC frequency control on the system protection of large-scale power grids is systematically analyzed. From the perspectives of ensuring the security and stability operation of the power grid and improving the efficiency of cross-region resource utilization, a coordinated control scheme of DC frequency control and emergency power support for integrating local and remote information of the power grid is proposed to achieve the coordinated operation between the system protections of large-scale power grids. Finally, the effectiveness of the proposed control scheme is verified based on actual power grid simulation examples.