Self-illuminating corner reflector is the corner reflector that all the rays that enter the reflector’s cavity experience the triple reflection on the panel and return to the radar. It has the advantage of reducing the unexpected coherent ground interaction and thus improving the accuracy of RCS and benefiting SAR calibration and image quality assessment. The optimal panel geometry is the one that has the minimal edge length for a given panel area in order to alleviate edge diffraction. The panel geometry had been previously designed to be square, pentagon and hexagon. In this paper, the general expressions for the panel area and panel external edge length of arbitrarily self-illuminating corner reflectors were described by parameter equation firstly. Then the edges of reflector panel were assumed as circular arc and further elliptical arc for analysis. Through a numerical approximation approach, the external edge lengths of those self-illuminating trihedral corner reflectors are derived and compared. The results showed that the self-illuminating trihedral corner reflector with circular arc had the minimal edge length and was the optimal panel geometry among the panel geometries under analysis.