An expanding device for a portable electronic apparatus is disclosed. The expanding device includes a main body for placing the portable electronic apparatus and a sliding assembly disposed in the main body. The sliding assembly includes a slide track element disposed in the main body, a sliding element, and a cover. The sliding element is slidably coupled to the slide track element and slides between a receiving position and an extending position relative to the main body. The cover is pivotally connected to the sliding element and includes a fixing portion. When the sliding element is moved towards outside of the main body to the extending position, the cover can be rotated relative to the sliding element, and the portable electronic apparatus can be fixed on the main body via the fixing portion of the cover.