The utility model discloses a reinforcement terrace reinforcing device for an existing building foundation. The reinforcement terrace reinforcing device comprises a reinforced concrete plate wall (1), and further comprises a newly increased terrace (3) and a wall passing equivalent rib (4). The wall passing equivalent rib (4) passes through the reinforced concrete plate wall (1) in a drilling manner. A double-layer bidirectional reinforcing mesh consisting of a newly increased terrace upper rib (5) and a newly increased terrace upper rib (6) is paved inside the newly increased terrace (3). The reinforcing mesh is anchored in the double-faced reinforced concrete plate wall (1).