Spatial layout automation is one of the research directions in intelligent CAD domain. The aim is to provide tools for designers to solve the layout problems emerged in building planning, facility planning, VLSI floor planning, product packaging and such like the fields. Human designers can interact with the tools and find some new creative solutions. The graph theoretic method is one of the main approaches in the research of spatial layout. One of the difficulties in layout design is the representation of layout knowledge and constraints. Former layout models can not fully represent layout knowledge and constraints. The drawbacks of former models lead the design process uncontrolled. This paper represents a normal-constraint-graph model and a hierarchical-constraint-graph model that can be used in 2D or 3D layout design. The two models combined can be used to represent the detailed knowledge and constraints in layout design. Thus, it can overcome the drawbacks of the former layout models. In this paper the condition of the existence of the solution of normal and hierarchical constraint graphs is discussed and the layout-plan-generation method based on the normal and hierarchical constraint graphs is given.