In view of the existing bad information diffusion models do not consider the information diffusion between different social networks, the connectivity principle in graph theory was used to establish a dynamic model of the diffusion of bad information among multiple social networks, and the optimal control theory was applied to the model. Through the optimal control principle, the existence of the optimal control strategy was proved. Then the optimal control model of the diffusion of bad information was obtained. The experimental results show that the introduction of optimal control measures can effectively suppress the diffusion scale of bad information, and the strength of the control strategy can be dynamically adjusted as needed. In addition, by simulating the conditions with and without information transmitted between different social networks, it is found that the information transmission between social networks will increase the scale and speed of the diffusion of bad information.