Cyber failures may cause severe power outages in a Cyber Physical Power System (CPPS). It is of significance to accurately assess the security of CPPS under cyber failure threat and implement a security defense of the system. First, this paper explains the research ideas on CPPS modeling and the cyber-physical interaction mechanism. Then, it establishes the framework of CPPS security assessment based on failure source analysis, failure spreading process analysis, and security metrics calculation. Finally, the CPPS security defense methods against cyber failure threat are summarized from aspects of information network structure optimization, routing optimization and cyber defense strategy, after which future research proposals are made. This work is supported by National Natural Science Foundation of China (No. U1866209) and Science and Technology Project of State Grid Zhejiang Electric Power Co., Ltd. (No. 5211JY180012).