A new method of computing bistatic RCS with Graphical Electromagnetic Computing is presented. The faces and edges, which are able to generate a diffracting field, can be detected by attaining the depth value of the faces of the targets relative to the incident direction and diffracting direction. The error of the Physical Optics field of edge, which is computed twice, has been corrected by using Physical Theory of Diffraction. The method of combining PTD with ECM is used in the disquisition. The result of using this method is the same as that showed in the documentation, which has proved that this method is effective.