While promoting the upgrading of traditional manufacturing, the industrial internet has introduced cyber security issues which makes industrial devices such as intelligent instruments face more severe cyber security risks. To effectively clarify and analyze the failure causes and hazards of intelligent instruments under functional safety and cyber security threats, this article proposes a method framework that enables reasoning and evaluation of integrated casual failure paths. Based on the framework, this article realizes the integrated failure analysis of functional safety and cyber security, and realizes the integrated causal failure path reasoning. Meanwhile, the importance of the failure path which represents the possibility of the path is quantitatively evaluated by attributes such as structure, probability and essence. Therefore, it realizes accurate analysis of instruments failure scenarios. Finally, the effectiveness and feasibility of the proposed method are evaluated by the intelligent transmitter. Through the failure path reasoning method based on dynamic causal diagram, it is the first time to reveal the influence principle of cyber security failure penetration between the internal functional modules of intelligent instruments, and realize the integrated failure analysis process of functional safety and cyber security.