Mechanical structure design is the most important process in the research and development of undersea equipment. Traditional design methods have limitations in obtaining functional demand information, determining the design direction, focusing on technical resources, guaranteeing systematic quality, etc. These limitations often reflect many problems, such as low design efficiency, waste of resources, and weak reliability of the finished products. By integrating the idea of grasping "the beginning " and "the end " into the process of mechanical design, this paper introduced a method called quality function deployment (QFD) and applied it to the mechanical design of subsea power devices to demonstrate and verify a robust design process with a clear direction and focused application of resources. Combined with the example of the mechanical design for the subsea power device, QFD was applied to analyse functional technical elements by establishing the house of quality (HoQ) models. And then, it was achieved a stable deployment and realization of the structural function for the subsea power device. Furthermore, for the key purpose of ensuring adequate structural strength and watertight reliability, this study selected suitable material, designed reasonable structures, and integrated finite element analysis technology to carry out mechanical simulations. Finally, the design of the mechanical structure for the subsea power device was robustly implemented.