A voltage-driven InP/InGaAsP 1 × 2 optical switch based on multimode interference (MMI) is proposed and numerically studied. In plane shift of the optical field across the output cross-section of a multimode waveguide enables optical switching between two symmetric output waveguides. Two twin electrodes on top of the multimode waveguide are tuned with reverse bias voltage to switch output port by reconfiguration of the refractive index profile. The optical switching mechanism and the design method are explained through a modal analysis of the wide perturbed waveguide. An optical switch design with an input waveguide width of 20 µm, output waveguide widths of 8 µm, and electrode length of 1.3 mm is proposed. The numerical prediction of the device performance is provided by combining a calibrated model of the voltage-perturbed refractive index and absorption with electrical and optical solvers. Simulation results show that an extinction ratio of 29 dB can be achieved at an optical wavelength of 1550 nm and with a switching voltage of −12 V.