Modular fixtures are used as flexible fixtures for holding a variety of workpieces in manufacturing processes where standard fixture elements are assembled into different configurations to adapt to different workpiece geometry. This paper presents a constraint-based modeling and automated design of modular fixture configuration design. Assembly constraints are defined in two levels so that the spatial relationships between fixture elements can be modeled and further inferred in terms of a set of constraints. The advantages of this method compared with our previous work include the associativity in fixture design. Once the constraints are specified, the assembly relationships are maintained even if other conditions may change, especially in the process of design modification. The constraint definitions and constraint satisfaction are presented. The application of the method to modular fixture configuration design is introduced.