The invention provides a method for using a width-adjustable chamfering crystallizer to cast an aluminum alloy slab ingot with a large width-to-thickness ratio and belongs to the technical field of metal processing. The method comprises the following steps of dosing, smelting, heat preserving, refining, standing, online refining, grain refining, filtering, casting and soaking. According to the method, in the casting process, the chamfering crystallizer is used, the width is adjustable, the equipment utilization rate can be increased, the equipment and maintenance cost is saved, the slab ingot milling amount is reduced, the yield is increased, the aluminum alloy slab ingot with the width-to-thickness ratio reaching 4.13-4.67 and the width size reaching 2270-2570 mm is obtained, and the requirements of different customers can be met.