A new polyimide (PI)-cantilever MEMS optical switch of electromagnetic actuation is proposed, and the structure of the optical switch is designed and simulated. The optimal dimension of the permanet magnet is designed and the structure of the PI-cantilever is optimized by finite element method. The relation of restoring force and electromagnetic force versus the displacement of the cantilever is interpreted. With 75 mA current pulse input,the distance of PI-cantilever equation is about 1.02 mm, which is able to change the propagation direction of the optical beam between through state and reflection state.