Abstract: The high-frequency method for the prediction of radar cross section (RCS) of complex conductive targets in the terahertz (THz) band in free space was presented. In order to consider the scattering fields of the complex perfectly electric conducting (PEC) targets in free space, the Green's function was introduced into the conventional physical optics method. Combining the graphical electromagnetic computing (GRECO) method and using the partition display algorithm to improve, the shadow regions were eliminated by displaying lists technology of OpenGL to rebuild the complex targets, and the geometry information was attained by reading the color and depth of each pixel. The THz RCS of complex conductive targets can be exactly calculated in free space. The RCS comparison between GRECO prediction and simulation of FEKO software proves the validity and accuracy of the proposed method. The results provides an important basis and method for the application of THz radar in many fields such as military, astronomy and remote sensing in the future.