In this paper, a new type 3×3 SOI multimode interference optical switch was presented. The index-modulated regions were introduceed in the multimode waveguide with the plasma dispersion effect (PDF) of silicon to change the regional index of the multimode waveguide, the phase of the light field was modified to determine the position of the output light field. The various states of the switch were simulated and analyzed using the finite difference beam propagation method (FD-BPM), the structure parameters of the switch was also designed and optimized. The peak extinction ratio of the switch reaches -17.27 dB with the optimized structure parameters.