We design a scalable networked control system of robot fish in 3-tier architecture using C# programming language under Microsoft. NET platform. The whole system consists of a webcam, a robot fish in pool, a wireless model, a local decision host and a remote controller. Real-time images of the robot fish caught by the webcam were sent to the decision host as well as the remote controller. The decision host processed the instructions from the remote controller and the real-time images of the robot fish synthetically, then sent control instructions to the robot fish via wireless model. An experiment result of the control effect of the system in Intranet was given.