Abstract: Based on parametric and semantic feature CAD systems，changing values of parameters is one of the most common operations，which is often a trial-and-error process.According to allowable parameter values are not known to the user beforehand，authors present a solution for automatically determining the allowable range for the parameter values.Authors find some distance or angle constraints by analyzing the graph of topological constraints and then put them into geometric constraints.Next，authors use the decomposition to find critical parameter values for sub-problems.By solving one problem instance for each interval between two subsequent critical values，the exact parameter ranges are determined for which a solution exists.