A horn antenna module is provided. The horn antenna module includes a waveguide and an antenna cover. The waveguide includes a plurality of waveguide paths. The antenna cover covers one end of the waveguide. The antenna cover includes a plurality of protrusions and a bottom surface. The protrusions are formed on the bottom surface. The protrusions correspond one-to-one with the waveguide paths. Each protrusion has a top surface and a lateral surface. The lateral surface is located between the top surface and the bottom surface. The top surface is planar.