Abstract: Terahertz wave is the last one which is not a comprehensive study of the development of the frequency window of the electromagnetic spectrum. It is of great scientific value and utilization of the terahertz wave. This paper presents the design of a 340GHz fourth harmonic mixer in terahertz band based on GaAs Schottky barrier diodes. By adopting soft-wares of HFSS and ADS, 3-dimensional modeling and analysis of the impedance-frequency characteristics are given to the anti-parallel diodes pair. Then the mixer was optimized. The mixer was also fabricated and measured. The measured results show that the conversion loss of the mixer is below 15dB in the band from 327GHz to 343GHz and the least is 12. 7dB.