The principle of hydrodynamics is applied to the design of automatic welding cover for underwater pipeline repair by using FLUENT software. The structure and size of the drain cover are determined,and the MIG automatic welding experiment is carried out for underwater pipeline repair. Argon is used as protective gas and the pressure of the experimental environment is 0.15 MPa. The velocity and pressure cloud images obtained through the calculation and simulation of the FLUENT software can directly show the influence of different intake modes on the airflow stability in the gas phase space. The underwater MIG automatic welding experiment by this method shows that the drainage hood has good drainage effect,the arc combustion is stable and the underwater weld quality is good,which lays the foundation for the underwater welding equipment and technology research.