Abstract: In order to improve efficiency of solving 3D geometric constraint systems for interactive assembly design,an online incremental solving method is proposed.In this method,the mixed graph is utilized to represent 3D geometric constraint system,and the equivalence analysis method is employed to reduce closed-loop subgraphs.Based on that,the dynamic update mechanism of geometric constraint mixed graph is established for interactive assembly design.Then,the directed broadcast of geometric constraint is computed to obtain the domain of influence by the change of constraint system.The core idea of the proposed method is to achieve the smallest solution of 3D geometric constraint system which is dynamically changed in interactive design process,so it can improve the computational efficiency.Finally,a typical example has been given to validate the correctness and effectiveness of the proposed method.