In parametric CAD design, the designers often encounter the question of judging whether a parametric model is under-, over- or well-constrained. In this paper, a graph-based algorithm for the question is proposed. This algorithm gives not only the decision of under-, over- or well-constrainess of a parametric model, but also the exact location where under-, over- or well-constraint occurs in the parametric model. This feature provides a more convenient way to the designers in the process of designing.