The solid-state harmonic mixer is vital to terahertz application system, which directly dominate the system performance. Simulation and design of a 670 GHz fourth harmonic mixer are described based on anti-parallel Schottky barrier diode with quasi-vertical structure. We use integrated simulation method based on harmonic balance algorithm to simulate and optimize the mixer, with the basis on the precise three-dimension model of quasi-vertical built in High Frequency Simulation Simulator (HFSS). Simulated result shows that under the Local Oscillator(LO) power of 10 mW in 167 GHz, the single-sideband conversion loss is less than 13.8 dB between 637-697 GHz of RF frequency. The minimum single sideband conversion loss is 10.6 dB at 679 GHz. The 3 dB conversion loss bandwidth is 60 GHz.