Aiming at the shortcomings of the existing multiplexers, a pattern of the asymmetric planar waveguide type mode division multiplexer/demultiplexer is designed, based on the effective refractive index matching effect. Waveguide structure consists of a main boom and two waveguide branches. The parametric principle to design the asymmetric planar waveguide type mode division multiplexer/demultiplexer is discussed. The performance of mode multiplexing/de-multiplexing is simulated under different waveguide width ratios, and the best waveguide width ratio is determined. The transmission process and the light field energy flow processes of fundamental mode and first-order mode are analyzed. The simulation results show that the asymmetric planar waveguide type mode division multiplexer/demultiplexer has a good performance of mode division multiplexing/demultiplexing, and the mode conversion can be realized efficiently. The properties, such as simple structure, high mode conversion efficiency, easy integration, make it more suitable for mode division multiplexing communication.