Abstract: We designed a terahertz radiometer antenna which is composed of an off-set Cassegrain reflector antenna system and quasi-optical feed network. This antenna system can respectively receive the electromagnetic signals at the frequency of 300 GHz, 380 GHz, and 425 GHz. Firstly, the operational principle of feed network and quasi-optical components is analyzed in detail. The simulated results of feed network are satisfied with the requirements for antenna system. Secondly, the whole antenna system of radiometer is discussed and the far-field radiation pattern of each frequency range is calculated. The results are well demonstrated that the radiometer antenna system has good performances in high gain, high beam efficiency, and high cross-polar level, which are much better than other systems. Lastly, the feasibility and rationality of this radiometer antenna system are well proved.