Abstract: GERCO(Graphical Electromagnetic Computing) is recognized as one of the most valuable methods for computing the high-frequency RCS (Radar Cross Section). To the complex radar targets, in order to obtain a satisfying result, the target geometry fitting is an important work in applying the GRECO method. This paper shows the principle method by using Catmull-Rom spline function geometry fitting theory, Visual C+ + language and OpenGL in Windows NT system. It gives the RCS curves of a standard body and a complex radar target, which are satisfying measured results. The results can have practical engineering worth.