A THz space-borne reflector antenna with an aperture of 2.5 m is presented. The antenna, which could receive electromagnetic signals at 183 GHz, is composed of the reflector antenna and quasi-optical feeding network. Its electrical performance is given, and the structural scheme is presented as well. The measurement of main beam efficiency achieves 90.2%, which could meet the specification of the GEO remote sensing satellites. It realizes a space-borne reflector antenna operating at terahertz frequency.