A sheet alignment device includes a first alignment arm, a second alignment arm, a first sensor and a controller. The first alignment arm includes a first blocking piece and a second blocking piece. During the sheet-aligning action of each sheet, the first alignment arm is moved for a preset distance from a standby position. Then, the first alignment arm is moved back to the standby position, and a waiting time is required to introduce a next sheet into the sheet placement tray. When the first sensor is no longer interrupted by the second blocking piece of the first alignment arm, the controller starts to calculate the moving distance of the first alignment arm. Since the moving distance of the first alignment arm is calculated at the same start point during the sheet-aligning action of each sheet is performed, the possibility of resulting in the moving distance error is minimized.