A novel 330 GHz sub-harmonic mixer based on anti-parallel Schottky diode pair is proposed. Microstrip circuit is used with waveguide-microstrip probe transition. Quartz glass of 50 μm thickness is adopted as the substrate, which is good for size-reduction. HFSS is utilized for 3D simulation first and then ADS for harmonic balance simulation. The simulation results show that the conversion loss is better than 9.5 dB form 310-350 GHz. The required Local Oscillator(LO) power is only 3 dBm, which reduces the demand on LO source.