Abstract: The layout design is a bridge to connect the conceptual design and the structural design of products．Taking the intelligent design system for the gear box of CNC plano milling machine as the background of study，the principles and methods of the automatic layout of the mechanical transmission system by computers are discussed．The layout design of products is divided into two levels．The layout-design-solving model that combines case-based qualitative layout with constraint-based parameter layout is presented．And a kind of data structures for a layout design——two-dimensional doubly linked lists——is put forward which can describe consistently all information related to the layout design and support the top-down model of the product design．