The images of a metal plate with a slit are obtained under various observation angles, by means of the method of moment and fast backprojection algorithm. Meanwhile, the impact on slit diffraction of edge diffraction and corner diffraction is eliminated through low reflection carrier design, image capture and RCS reconstruction method. Then, the slit RCS on a metal plate versus incident angles is given, and the variation of the slit RCS on a metal plate versus frequency, polarization, slit width and plate thickness are analyzed, which provides a basis for the low reflection target RCS prediction and suppression.