The layered algorithm of stereolithography (STL) file is the most basic and most important thing for Three Dimensional (3D) printing. A new optimal algorithm based on MTALAB software is proposed to guarantee the accuracy and stability based on the existing algorithm. The implement process of method was firstly，the triangles were sorted to find in a layered triangle；secondly，the adjacency relations of these triangles were built；thirdly， the intersection point was found that can be directly extracted contour lines. The algorithm uses the most basic programming code in MATLAB software，which concisely and effectively extracts the outline of the ring under the condition of accuracy. An experiment is done to verify its stability and accuracy.