In this study a methodology of computer aided conceptual design in product development based on ergonomics was put forward. When products concerned with people are designed, ergonomics should be taken into account. The digital human model was used to decide the layout of design scheme. Ergonomics features were extracted in workplace analysis. With the aid of software Pro/Engineer, the whole human and machine virtual environment were built. A case of the modeling and size specification of the excavator cab for different drivers was given as the example to show the whole design process. The results shows that the development of ergonomics, computer aided concept design and anthropometry can help the designers to improve product development.