Method for adding underground parking lot under existing building

The invention discloses a method for adding an underground parking lot under an existing building. The method comprises the following steps that retaining walls are formed on the opposite sides of theexisting building, and working pits are dug in the retaining walls, wherein the main working pit is formed in one side, and the auxiliary working pit is formed on the other side; the main working pitis used as an initial work site, multiple rows of prefabricated members are sequentially and laterally pushed to the lower part of the existing building in sequence, reinforcing cages are correspondingly laid in the rows of prefabricated members, concrete is poured, and multiple reinforced concrete underpinning beams are formed; two side beams are formed at both ends of each reinforced concrete underpinning beam, and an integral underpinning structure under the existing building is formed; extraction is carried out from one side of the main working pit to the lower part of the integral underpinning structure, and a pa