Abstract: This paper introduces a new approach to constructing descriptive 3D geometric constraint models for the B-rep models composed of analytic surfaces.Based on the principle of decoupling for the degrees of freedom of position and posture,the geometric representations of analytic surfaces are first presented.Then, according to the perpendicularity or parallelism of spatial vectors,several elementary constraints are defined, and the algebraic equations for some general engineering constraints are derived.After analysing the process of automatically constructing descriptive geometric constraint models,a prototype software package for directly editing 3D shapes is developed to verify the feasibility of the proposed method.