A 300 mm offset-feed reflector antenna is designed for a W-band radiometer in this paper, its parameter are 94.5 GHz operating frequency, sidelobe level of less than-25 dB, cross polarization level of less -30 dB and half-power beamwidth of better than 1 °. A smaller offset angle is adopted to improve the antenna cross polarization characteristics, and this matching feed is designed with the focal plane field matching method and the modulus ratio method and is optimized to get good VSWR and lobe equalizing level. Measurement results show that this antenna meets the requirements of the radiometer and its measured and theoretical values are coincided.