A fixing device includes a substrate, a slot, a fixing bracket. The slot is disposed on the substrate, and a first end of an expansion unit is inserted in the slot. The fixing bracket is disposed on the substrate, in which the fixing bracket includes a force receiving upper portion and a supporting lower portion. An accommodating space is defined between the force receiving upper portion and the supporting lower portion. When a force is applied to or released from the force receiving upper portion, the force receiving upper portion elastically deforms or elastically recovers. Therefore, the fixing bracket switches between an open state and a fixing state for enabling the accommodating space to selectively accommodate a second end of the expansion unit.