In this paper, the shape variation law of Bezier surfaces, B-spline surfaces and NURBS surfaces (BBN surfaces) is investigated under the modification of a control point of the surfaces. The relationship between the geometric elements of the surface before and after deformation and the deforming displacement is built up by decomposition a BBN surface into the summation of a convex surface and a surface with special shape. The law of a convex BBN surface losing its convexity is also obtained. The achievements in this paper can guide the algorithm design and shape control in CAD modeling.