A design method and experimental results of the low loss terahertz cavity used in terahertz molecular clock are introduced. Physical simulation design and structure design of terahertz cavity are carried out by CST. Through simulation and optimization, the miniaturized terahertz cavity structure with the reflection coefficient S 11 less than -10 dB is designed in the range of 230 GHz±10 GHz. The fabrication of terahertz cavity is completed by using the existing processing technology. The reflection of the cavity in G band is less than -10 dB and the transmission loss is less than -5 dB, which lays the foundation for the development of terahertz molecular clock.