An accessory suitable for an electronic device is provided. The electronic device has a display area. The accessory includes a coupling portion and a cover portion. The coupling portion is suitable to be coupled to the electronic device. The cover portion is connected to the coupling portion and is suitable for covering the display area of the electronic device. The cover portion has a plurality of light-transmitting areas, and the light-transmitting areas are arranged on the display area in an array. An image generated by the display area is projected out of the cover portion through the light-transmitting areas. Moreover, an electronic assembly containing the electronic device and the accessory is also provided. Furthermore, a controlling method is also provided for controlling the electronic assembly.