Abstract: A multiple-knot method based on a non-uniform B-spline is adopted to design and fit complex combined curves.We have solved the problem of reducing the order in a linear equation set for varieties of multiple knots in inverse calculation for the vertices of a characteristic polygon and obtained an unified expression.The program provided in this paper is flexible and convenient for fitting,designing and modifying complex combined curves quickly in every possible case.This method can offer a more universal tool for using the B-spline in computer-aided geometric design and drawing.