A sub-harmonic 0.45 THz mixer is optimized and designed with quasi-vertical Schottky mixer diodes of ACST. A circuit topology which is compact and easily to be matched is proposed. Based on the diode structure analysis and modeling, the mixer circuits and structure are optimized as a whole by using an RF circuits simulation software(ADS) and an electromagnetic field simulation software(HFSS). The simulation results reveal that minimum Single Side Band(SSB) conversion loss is about 6 dB with 3.8 mW of local oscillator power, and -3 dB bandwidth is more than 30 GHz. The circuit topology is suitable for the use of anti-parallel Schottky diodes to achieve sub-harmonic mixer designs.