The fast, accurate, and effective design of high-performance terahertz devices is very significant for the advancement of terahertz communication systems. However, the traditional methodologies encounter big challenges, such as numerous parameters and complex nonlinearity in device design. In this work, an innovative, AI-assisted approach harnessing refined swarm intelligence (SI) optimization algorithms were introduced for the design of a 220 GHz wideband subharmonic mixer. Simulation results confirm that, with a local oscillator frequency of 110 GHz maintained, the operating frequency range is 200-240 GHz, a 40 GHz RF bandwidth has been achieved, displaying a single-sideband conversion loss (CL) superior to 7 dB and the best CL of 6.15 dB. This accelerates the development of cutting-edge terahertz devices and catalyzes significant progress within the terahertz research fields.