The structure of parts produced by additive manufacturing(AM) technology is complex, and usually with overhanging feature structure and symmetric structure. To ensure the mechanical property of symmetric structure and avoid the molding defects caused by overhanging feature structure, it is necessary to extract the feature structure from the part model for special process control before molding. This paper combines the layer upon layer characteristics of part model and the grid information of STL model, then proposes a calculation method of the cantilever inclination angle to effectively identify the overhanging area; and design the box of space surface to accurately extracts the structure and accomplish the operation by 3D modeling software. Finally, a typical symmetric part verification method is selected, and printed it out with better molding quality by SLM additive manufacturing technology. The results show that the method is feasible and effective, and improve the processing capacity and formed quality of parts.