Flattening of freeform surfaces (i.e., non-developable surfaces) is a difficult problem in engineering application. According to flattening distortions, a new development based on energy model is put forward in this paper. Given a doubly curved surface and using a triangle mesh subdivision, some triangles will be distorted when the surface is flattened. Generally, the developed plane will possess many splits. There are two kinds of developments: unconstrained flattening and constrained flattening, which are both analysed. In order to obtain a better pattern, the strain energy required to force a 3-D shape adopt the plane pattern should be minimised. At last a flattening algorithm is developed on the basis of theoretical analyses. Compared with the others, the proposed method can control the local precision easily, and is an effective method for flattening doubly curved surfaces.