We propose a polarization insensitive multimode interference coupler (MMI) design for optical 90° hybrid. The 90° hybrid used in coherent receiver application is based on the Indium Phosphorus (InP) platform, which can realize monolithic integration with detectors. By using the three dimension beam propagation method, a 90° hybrid based on a polarization insensitive MMI has been designed and optimized. We find that there is an ideal interference length for both transverse electric (TE) mode and transverse magnetic (TM) mode in this structure. Using the designed 90° hybrid, we demonstrate the common mode rejection ratios for in-phase channels and quadrature channels better than -20 dB and the phase errors better than ±3° in an ideal interference length range. The phase errors of the I-channel and Q-channel less than ±4° when the interference length is 480μm across the C band (1535-1560 nm).