To identify the vulnerabilities of Cyber Physical Power System (CPPS) and formulate the corresponding prevention and control strategy, the models of two kinds of CPPS interdependent networks with different subnetworks were presented, in which IEEE 118 bus system was chosen as Power Grid and cyber networks adopted two typical structures: double-star and mesh, respectively. An index of comprehensive importance on interdependent pairs of nodes was proposed, and it was used to identify the key pairs of interdependent nodes causing large influence on CPPS structural vulnerabilities, which overcomed the limitations of single-layer network indicators. Based on the model of cascading failures in interdependent network, CPPS structural vulnerabilities were analyzed by using two extreme attack strategy: randomly attack and deliberately attack. The effect on CPPS structural vulnerabilities of Key Node Protection Strategy was considered. Simulation results indicate that, compared with double-star cyber network CPPS, the structure of mesh cyber network CPPS is more vulnerable under randomly attack. Under deliberately attack strategy, prior attacking pairs of interdependent nodes can cause more damage to network connectivity. CPPS structural vulnerabilities can be improved with selecting proper nodes of Key Node Protection Strategy.