The invention discloses an underpinning device for an ultra-wide wall body and an underpinning method thereof. A hole is drilled in the bottom of a wall by using a drilling machine, and a steel pipe is inserted into the hole; the two ends of the inserted steel pipe extend out of the wall body; the steel pipe wall is reserved with a grouting hole; cement paste is pressed into one end of the steel pipe; after the steel pipe is full of the cement paste, the cement paste is pressed into a gap between the steel pipe and the drilled hole through a grouting hole under the action of pressure in order that the steel pipe is tightly connected with the wall. Along the direction of the wall body, steel pipes are arranged at certain intervals, wall clamping beams are constructed on the two outer sides of the wall body after all the steel pipes are filled with the cement paste, and the two ends of the steel pipes are poured into the wall clamping beams, so that the wall body, the steel pipes and the wall clamping beams are fixedly connected into an underpinning whole. Jacks are arranged at the bottoms of the wall clamping beams for jacking, so that an upper wall body can be underpinned onto a lower slipway beam or a newly-made foundation.