This letter presents a new port-assignment model of an anti-parallel Schottky barrier diode (AP-SBD) and a 0.3-THz sub-harmonic mixer (SHM) as the application design. Using the proposed method, the AP-SBD is accurately modelled compared to conventional modelling methods. Because the characteristics of an AP-SBD have the greatest impact on mixer performance, an accurate modelling method for AP-SBD helps design SHMs with good performance. To verify the proposed modelling in the terahertz region, a 0.3-THz SHM based on the modelled AP-SBD is designed and assembled. The measurement result reveals that the optimal double-sideband (DSB) conversion loss of the mixer is 6.7 to 10.1 dB within 279 to 318 GHz when the fixed local oscillator (LO) signal is 6.4 dBm at 152 GHz. All measurement results confirm that the proposed methodology is concise and feasible for the design of terahertz SHMs.