The invention relates to a method for constructing an underground garage, which has the characteristics that: structural stability of the original building, high construction speed and low cost are guaranteed. According to a technical scheme, the method for constructing the underground garage under the old building is characterized by comprising the following steps that: a, an underground diaphragm wall or a sheet-pile wall is poured at a position which is at least 1m away from an exterior wall around the building; b, a group of cross beams are poured at the foundation at the bottom of an overhead layer of the building, and the cross beams are integrally connected with foundation soil, the underground diaphragm wall or the sheet-pile wall to support the building; c, underground space is dug below the foundation soil of the building, an entire plate foundation layer is poured at the bottom of the underground space, and the entire plate foundation with underground piles and the underground diaphragm wall or the sheet-pile wall are integrally connected; d, a part of the underground piles are cut off between the foundation soil and the entire plate foundation and the rest underground piles are reinforced; and e, a slope is arranged on the ground and the slope is communicated with the underground space to be served as a passage for entrance and exit of vehicles.