A broadband 360~440 GHz subharmonic mixer based on Schottky diode designed by UESTC and fabricated by CETC-13 is presented. Diode modeling is described to simulate the electromagnetic environment due to geometry-dependent parasitic effects in the extremely high frequency band. The whole subharmonic mixer is optimized by means of combination of field and circuit in HFSS and ADS. Measured results show that the sub-harmonic mixer yields minimum conversion loss of 9.99 dB at 406 GHz, while the conversion loss was less than 15 dB over 380 GHz to 430 GHz and less than 19 dB over 360 GHz to 440 GHz when LO power was 6 dBm at 210 GHz.