In order to fill the gap of the underwater robot at shallow sea area, an underwater detecting robot based on pneumatic technology was designed and put forward. The robot was equipped with underwater camera which could be used as realtime monitoring of underwater environment and fish growth at inshore cultivation. Stable dynamic performance was required when the underwater vehicle was moving. However, the robot motion precision could be affected due to the everchanging underwater work environment. A closedloop control system was designed for the diving process of the robot, and the velocity deviation was corrected through the dynamic feedback. The composition and working principle of the closed loop control system were introduced. Based on this, the transfer function of each part and the whole system are deduced, adopting proportion integration and differential (PID) to regulate system parameters so as to make sure the feasibility and stability of the system.