We proposed and demonstrated the ultra-compact 1 310/1 550 nm wavelength multiplexer/demultiplexer assisted by subwavelength grating (SWG) using particle swarm optimization (PSO) algorithm in silicon-on-insulator (SOI) platform. Through the self-imaging effect of multimode interference (MMI) coupler, the demultiplexing function for 1 310 nm and 1 550 nm wavelengths is implemented. After that, three parallel SWG-based slots are inserted into the MMI section so that the effective refractive index of the modes can be engineered and thus the beat length can be adjusted. Importantly, these three SWG slots significantly reduce the length of the device, which is much shorter than the length of traditional MMI-based wavelength demultiplexers. Ultimately, by using the PSO algorithm, the equivalent refractive index and width of the SWG in a certain range are optimized to achieve the best performance of the wavelength demultiplexer. It has been verified that the device footprint is only 2×30.68 μm², and 1 dB bandwidths of larger than 120 nm are acquired at 1 310 nm and 1 550 nm wavelengths. Meanwhile, the transmitted spectrum shows that the insertion loss (IL) values are below 0.47 dB at both wavelengths when the extinction ratio (ER) values are above 12.65 dB. This inverse design approach has been proved to be efficient in increasing bandwidth and reducing device length.