A three-dimensional optical switch is introduced which is composed of two N×N three-dimensional multi-mode interference couplers and an array of phase shift waveguide. Firstly, the general imaging principle of three dimensional multimode interference is analyzed by the guided-mode propagation method，and the imaging position and phase matrix are deduced. And the transmission equation of optical switch is set up by the transmission matrix method.The phase condition when the optical switch works is calculated by the transmission equation,and verification and the simulation is completed by three-dimensional finite-difference beam propration method. The insertion loss and crosstalk when the optical switch works are given, and the relation between the output power and the phase shift of phase array waveguide are obtained.