Abstract: An algorithm providing the spacecraft geometric description for vulnerability assessment is proposed. The original model preparation for the algorithm may be approximated by a series of cylinder, sphere, and cube segments, or by reading quadrilateral finite elements from finite element data files built in MSC/PATRAN. The basic principle to calculate projected area, overlapping area from any aspect considered is shown in this paper, which uses the vertical scanning line and shot line methods. The intersection points of shot line and quadrilateral finite elements in the overlapping area are recorded to determine the masking/shielding among components. The proposed algorithm has been programmed to offer the sufficient detailed target description and application shows that it can deal with any muti-overlapping among components and the error can be reduced through shortening the step length.