A novel high-frequency radar cross section prediction method is introduced to solve the scattering from electrically large conductive target. It can be realized by combining a modified-vector physical optics algorithm and a graphical electromagnetic computing algorithm. In consideration of the traditional physical optics algorithm which needs to compute the edge diffraction, the radar cross section of the target cannot be calculated directly, but it is needed to first calculate the diffraction contribution, then to add the scattering contribution, therefore the radar cross section of the target can be finally obtained. By using the improved graphical electromagnetic computing, the surface normals, thus the surface current, can be directly corrected. Therefore the diffraction at the edges is taken into account, thereby improving the efficiency of the algorithm. The central processing unit running time involves only the time needed to compute the electromagnetic part, with leaving the geometric part to the graphics hardware. The line integrals are computed over an arbitrary shape in real time. Numerical results demonstrate the good accuracy and efficiency of the modified method.