This paper presents a method to judge whether a facet is shadowed by another in calculation of radar cross section (RCS) utilizing physical optics (PO). The complex electric large object (CELO) is discretized into a huge number of triangle facets. This paper assumes that if the center of the facet is shadowed, then the whole facet is discarded. In the shadowing judgment, determinants consisting of three vectors are calculated and the sign can indicate the shadowing relation. This method avoids the intersection calculation and depth comparison that have large calculation cost. At the end, some numerical examples validate the accuracy and efficiency of the algorithm.