In order to reduce the resistance caused by the adhesion of Marine organisms to the ship sailing and improve the transport efficiency of the ship, a underwater ship hull cleaning ROV based on six thrusters is designed. Firstly, the corresponding software and hardware scheme of the robot was designed, and the driving and measuring components of the robot were selected according to the actual application requirements. Then, the structure of the ROV was designed using Solidworks 3D mechanical drawing software, and the structure assembly and system test of the ROV prototype were carried out. At the same time, in order to improve the performance index of ROV attitude control process of ship cleaning, an improved variable integral PID control algorithm is proposed. According to the change of the amount of deviation, the whole attitude control process is planned into different stages, and the switching coefficient of the integral term is adjusted to meet the needs of different stages. Finally, the improved ROV attitude control system was simulated and tested by MATLAB simulation software. The results show that the algorithm has shorter transition time and better stability than the traditional single closed loop and double closed loop PID control algorithm, and it is a stable and efficient ROV attitude control mode.