In order to solve the problem that network security attacks against the Cyber-Physical System (CPS) will cause a system failure, a CPS risk modeling and analysis method based on dynamic fault tree was proposed. Firstly, the integrated modeling was performed to dynamic fault tree and dynamic attack tree to build the Attack-Dynamic Fault Trees (Attack-DFTs) model. Then, the formal models of static subtree and dynamic subtree in Attack-DFTs were given by binary decision graph and input-out Markov chain respectively. On this basis, the qualitative analysis method of Attack-DFTs was given to analyze the basic event path of the system failure caused by network security attacks. Finally, the effectiveness of the proposed method was verified by the typical case study of a pollution system. The case analysis results show that, the proposed method can analyze the event sequence of system failure caused by network security attack in CPS, and effectively realize the formal safety assessment of CPS.