The boundary mesh of the casting model was determined by direct calculation on the triangular facets extracted from the STL file of the 3D model.Then the inner and outer grids of the model were identified by the algorithm in which we named Inner Seed Grid Method.Finally,a program to automatically generate a 3D FDM mesh was compiled.In the paper,a method named Triangle Contraction Search Method (TCSM) was put forward to ensure not losing the boundary grids; while an algorithm to search inner seed grids to identify inner/outer grids of the casting model was also brought forward.Our algorithm was simple,clear and easy to construct program.Three examples for the casting mesh generation testified the validity of the program.