The invention discloses a three-dimensional segmenting method for an STL (Standard Template Library) triangular network model in rapid molding process. The three-dimensional segmenting method comprises the following steps of: firstly, reading the STL triangular network model from an STL file into an internal storage, and establishing complete topological information of a surface patch, a side and a peak in the STL model based on an improved wing-edge structure; secondly, repeatedly searching through information in the internal storage to obtain a highest position surface patch serving as a seed surface patch in an original STL model, and searching an adjacent surface patch having the same normal vector Z-axis component symbol as the seed surface patch with a boundary expansion method on the basis to obtain a plurality of sub-STL models having the same normal vector Z-axis component symbol; and lastly, checking the boundary of each sub-STL model, recognizing and extracting covered boundary lines, and segmenting the sub-STL model by using a vertical plane defined by the covered boundary lines to further generate non-interfered sub-STL models. By adopting the three-dimensional segmenting method, three-dimensional segmentation of the STL triangular network model can be realized effectively; and moreover, the triangular network model has the advantages of high adaptability, high efficiency and low resource consumption.