A new solving method for geometric constraint reasoning is presented in this paper which has been named as maximal-reduction algorithm (MRA). It is the application about constraint network theory, analysis of degree of freedom, sparse matrix, and graph theory. With the MRA, geometric constraint system can be transformed into a series of subsystem which are organized with a forest data structure finally. The strategy enables the time complexity to solve the system separated with the scale of the system. It improves the solving stability and time expenses greatly. An interesting character of the MRA is its generalization which gives itself wide applications including parametric drawing, parametric modeling, kinematics of multibody system and product assembly.