A novel constructive algorithm is presented to model free-form surfaces based on general skeletons in this paper. Each skeleton is first used to construct a distance field, then a smooth implicit surface is generated by using the implicit function blending and CSG (constructive solid geometry) representation technique. The final surface mesh is achieved by polygonizing the implicit function. By using the general skeletons as primitives, the authors break the limitation of traditional implicit method which uses points as primitives. The distance surface fits the shape of the original skeleton well, which makes users be able to interactively design complex surfaces intuitively. Furthermore, the introduced warping function greatly enriches the modeling means. The experimental results demonstrate that the prototype system based on this algorithm can conveniently and intuitively construct complex free-form surfaces.