Assembly information is important in enterprise information systems. Assembly of the car body is flexible. The paper presents non-forced decomposition of the car side aperture and achieves dimensional adjustability. At the same time use polychromatic sets to build the relational model of assembly of the car body and propose the mathematic form of these models, namely locating relation equations, which express the constrained relationship of locating, and displacement interference equations, which express the constrained relationship of possible displacement. The algorithms of obtaining equations are proposed. To validate the method, take the car side aperture as an example.