Abstract: Parametric design plays an important role in the modern CAD technology． Parametric model constructing，management and solving of the geometric constraint system is the key of parametric design．This paper describes a new geometric reasoning method based on the dynamic reduction of the geometric constraint network．While constructing the parametric model，the geometric constraint system obtains the maximal decomposition． A reducing tree is used to represent the reduction result，which explicitly expresses the coupling degree of the constraint system and the solving order．In implementation， the algorithm is very effective for geometric constraint consistence checking、 fast solving and etc．