The invention discloses a multi-mode interference optical switch with the controlled input position. The multi-mode interference optical switch comprises a plane optical wave optical path chip, a light source, an input optical fiber array and an electric and automatic core adjusting device. The plane optical wave optical path chip comprises a silicon substrate, a silicon dioxide buffer layer, a waveguide and a covering layer. The waveguide comprises an input-stage single-mode waveguide, a middle-stage multi-mode waveguide and n output-stage single-mode waveguides. The light source is connected with the input end of the input optical fiber array. The output end of the input optical fiber array is connected with the input end of the input-stage single-mode waveguide. The output end of the input-stage single-mode waveguide is connected with the input ends of the n output-stage single-mode waveguides through the middle-stage multi-mode waveguide. The input optical fiber array is fixedly connected to the electric and automatic core adjusting device. The on-off mode of the optical switch is adjustable, the technology and the structure are simple, and the cost is low. The invention further discloses an adjusting method of the multi-mode interference optical switch with the controlled input position. The adjusting method is simple and capable of rapidly adjusting the on-off mode of the optical switch.