For manufacturing a light-guiding plate and a key module having the light-guiding plate, a mold is prepared first. Then, a material having high light transmittance is injected into the mold. After the plastic material is formed, the mold is released, so that the plastic material is formed into a sheet-like light-guiding plate, and one surface of the light-guiding plate is formed with a plurality of light-guiding portions thereon. The light-guiding plate is disposed in the key module of an electronic device. The light generated by light-emitting elements within the electronic device is introduced from the front end of a substrate of the light-guiding plate, and the light is refracted by the light-guiding portion to a corresponding key, thereby to display the numeral or pattern of the surface of a key cap of the key.