The invention discloses an inner-support-free double-wall locking mouth steel bushing box cofferdam construction method and an inner-support-free double-wall locking mouth steel bushing box cofferdam. Firstly, unit blocks are processed in a factory and then transported to a construction region to be spliced and formed piece by piece, the spliced unit blocks are wholly hoisted to a target pier position at the preset elevation through a large hoisting ship, then construction steps of sludge suction, tenon type locking mouth plugging, sand cushion layer filling, concrete bottom sealing, water suction, foundation clearing, steel protection barrel cutting, pile head removing, whole abutment installation and the like are performed, finally whole dismantling is performed for usage of the next pier position construction. The inner-support-free double-wall locking mouth steel bushing box cofferdam comprises a side plate, a blade foot and a joint and is characterized in that the cofferdam is of a detachable structure, a plane is partitioned into blocks, the blocks are connected in a mode that tenon type locking mouths are combined with bolts, and the inner side is not provided with an inner support. Therefore, the inner-support-free double-wall locking mouth steel bushing box cofferdam has the advantages of being small in construction difficulty, short in construction period, low in cost input, small in material consumption, high in repeated usage times, capable of being wholly recycled, safe, reliable and the like.