The invention discloses an anti-settlement device for a foundation of an additionally-mounted elevator and a construction method thereof. The anti-settlement device comprises anti-settlement piles, ananti-settlement box, a rubber box, a rubber cover plate, an anti-settlement cover plate, an anti-settlement column, spring columns, elastic fillers, first connecting pieces and second connecting pieces. The structure is simple, very common materials are adopted, the construction cost and the material cost are low, construction is fast, normal life needs of people are not affected, practicality ishigh, and popularizing is easy; connecting between an elevator shaft and the elevator foundation are well reinforced, when the elevator shaft moves, the quite good buffering effect is achieved, the cooperative work effect is generated between the shaft and the foundation, the good using effect can be achieved whether lateral moving or vertical moving, the quite good reinforcing effect is achieved, and the traveling safety of people is guaranteed.