A 90° optical hybrid based on an InP 4×4 multimode interference waveguide (MMI) was designed and fabricated. The optical hybrid was fabricated with InGaAsP waveguide layer, InP cladding layer and InP substrate. The width of the single mode waveguide was set to be 2.6 μm, and the length and width of MMI were set to be 844 and 20 μm, respectively. Three dimensional beam propagation method (3D BPM) was used to simulate and analyze the process error tolerance of refractive index, thickness, width and length of wavwguide materials. It is found that optical hybrid’s insertion loss is less than 1 dB and phase deviation is less than 5° in the wavelength range of 1 535~1 565 nm, which is consistent with the simulation result.