A sub-harmonic monolithic mixer with a center frequency of 0.825 THz is developed based on GaAs monolithic microwave integrated circuit technology. The parasitic parameters of the anti-parallel Schottky diode at the terahertz frequency are analyzed to improve the circuit design. The monolithic circuit is suitable for terahertz devices with the characteristics of high integration and little fabrication deviation. Meanwhile， the beamlead circuit is used to reduce the loss of substrate and installation position offset. Measured results show that the single-sideband （SSB） conversion loss of the mixer is lower than 33 dB in the frequency range 0.81～0.84 THz， and the minimum SSB conversion loss is 28 dB.