The invention relates to the technical field of immersed tunnels, and specifically relates to an M-shaped water-stop belt and a mounting method. The M-shaped water-stop belt comprises a first connecting part which is connected with a final joint body structure, and a second connecting part which is connected to a pushing joist, wherein a telescoping part which is manufactured through a flexible material is arranged between the first connecting part and the second connecting part; when the pushing joist is operated to push, the telescoping part is M-shaped, and the first connecting part, the second connecting part and the telescoping part are integrated; the M-shaped water-stop belt is correspondingly fixed to the final joint body structure and the pushing joist through the connecting parts; an empty chamber structure, at the inner side, of the pushing joist is separated from seawater at the outer side through the M-shaped water-stop belt. Therefore, the water stopping purpose is achieved; the seawater is prevented from entering the empty chamber structures of the pushing joist and the final joint, and as a result, a final joint can be smoothly mounted and safely constructed.