A highly compact 2×2 multimode interference (MMI) coupler based on silicon-on-insulator that can be used in an array waveguide grating demodulation integration microsystem is designed. The coupler is simulated using the beam propagation method. Taper waveguides are used as input/output waveguides. The footprint of the MMI region is only 6μm×57μm. The excess loss is 0.46dB, and the uniformity is 0.06dB with transverse electric polarization when the center wavelength is 1.55μm. The maximum excess loss is 1.55dB in the range of 1.49μm–1.59μm. The simulation results show that a small 2×2MMI coupler exhibits low excess loss, wide bandwidth, and good uniformity suitable for the requirements of the system on chip.