We design the 2×2 multimode interference (MMI) coupler based on silicon-on-insulator (SOI), which can be used in the array waveguide grating (AWG) demodulation integration microsystem. The coupler is simulated using the beam propagation method (BPM). Taper waveguides are used as input/output waveguides. The footprint of the MMI regions is only 6 μm×57 μm. The excess loss is 0.46 dB and the uniformity is 0.06 dB with TE polarization when the center wavelength is 1.55 μm. The maximum excess loss is 1.55 dB in the range of 1.49~1.59 μm. The simulation results show that the 2×2 MMI coupler with small size exhibits low excess loss, wide bandwidth and good uniformity, which can meet the requirement of system on chip.