With the step development of UHVDC construction in China, the stability problem of the receiving power grid has become more prominent than before, and the frequency stability problem is particularly important. In this context, this paper studies the deterioration of system protection frequency control performance under the scenario of node failure and information transmission failure. On the basis of the output of considering DC coordinated control, cutting pump control, excision interruptible load control and conventional units, then do some risk simulation on IEEE-39 node system and defined the risk index and risk level of the system protection frequency control .By comparing the risk index and risk level of each node, the large-capacity generator nodes, topological connection nodes, and heavy-duty nodes are selected as important nodes. At the same time, this paper proposes four mitigation strategies, such as DC coordinated control, cutting pump control, excision interruptible load control and node protection, to reduce the risk of Cyber-Physical Power System. It is verified that the protection of important nodes can effectively reduce the impact of node failure and information transmission failure on system protection frequency control performance. The conclusion proves the effectiveness of the risk grading and high-risk screening of Cyber-Physical Power System and the corresponding mitigation strategies for reducing the system risk.