Abstract: To facilitate designers to create and or modify 3D shapes rapidly during prototype modeling, this paper presents an online interactive approach which can easily generate 3D composite surface of various shapes.The approach incorporates the sketch-based interface into the construction of variational implicit surface.It infers 3D surface constraint points from 2D drawing strokes (including contour stroke, cross-section stroke and skeleton stroke) using inflation transformation, cross transformation and bend transformation respectively, users can then control the shapes of 3D surfaces with freehand sketches.A technique of localization of variational implicit function is adopted for real time surface blending.Experiment results show that the proposed method supports the “divide-and-conquer” construction process of composite surface efficiently.