In order to increase efficiency and accuracy in auto-segmentation of STL dental cast for invisalign, the paper improves the traditional segmentation algorithm. The new algorithm changed single piece input mode into area selection input mode . The bending degree expression about adjacent surfaces was taken as the corresponding height field functions. We designed the direct insertion sort instead of the stack sort. The result shows that the new algorithm can achieve a more complete segmentation for dental crown and improve efficiency, as meets invisalign requirements.