With the development of virtual prototyping technology, computer simulation has been used in product design as checking and optimizing product design. The users can test product model, simulate test and estimate product by means of digital analyzing technology. They can improve product efficiency and reduce cost when they improve design. Digital simulation design is one of the mechanical product modern design methods. The authors have introduced the concept and meaning of digital mockup technology in mechanical design and its architecture, design cycle, which are concerned multi-domain UML, multi-body dynamics and multidisciplinary design. They also have analyzed the automobile digital simulation design method and digital mockup technology.