According to the coupled mode theory,a three-core fiber based three-mode multiplexer/demultiplexer supporting to transmit three spatial modes(LP_(01),LP_(11a),LP_(11b))is designed.This three-core fiber consists of a threemode core located in the center and two single-mode cores deployed in the outer layer.The structural parameters of the three-mode core are chosen,and then according to the effective refractive index matching principle of mode,parameters of two single-mode cores in the outer layer are design...