A spacing interval control method is used in a sheet laminating apparatus for controlling a constant spacing interval between any two adjacent ones of multiple documents. The sheet laminating apparatus includes an automatic document feeding mechanism and a sheet laminating mechanism. By means of multiple sensors within the automatic document feeding mechanism, multiple documents are successively transported from the automatic document feeding mechanism to the sheet laminating mechanism at a constant spacing interval between any two adjacent ones of multiple documents. As a consequence, the documents after being heated or pressed by the sheet laminating mechanism will be cut into multiple separate and identical-sized laminated documents.