The invention relates to the technical field of a shield tunneling machine, in particular to a rapid expanding device for the shield tunneling machine and an expanding method. A reinforcing circular plate A and a reinforcing circular plate B are welded to the two ends of an original shield body, an expanding shell is welded between the reinforcing circular plate A and the reinforcing circular plate B, a concrete is poured between the expanding shell and the original shield body to fill a gap between the expanding shell and the original shield body, so that the whole rigidity requirement of theshield body is guaranteed, the use level of a steel material is low and is greatly reduced, so that the expanding cost is effectively lowered; in addition, the quantity of soldering points between the rapid expanding device and the original shield body is small, so that the expanding shell is easy to dismount, and can be recovered to the shape of the original shield body easily, and the originalshield body can be repeatedly used.