The invention relates to a dismantling method for an inner supporting beam of a foundation pit. The dismantling method comprises the following steps of: erecting a temporary supporting structure on the bottom of the inner supporting beam of the foundation pit for supporting the inner supporting beam; constructing on the end part of the inner supporting beam to form a fracture for separating connection of the inner supporting beam and a water cut-off curtain, and keeping a bottom-layer main rib on the fracture while forming the fracture; dismantling the inner supporting beam and keeping the bottom-layer main rib, and blocking vibration waves formed by dismantling from transmitting to the water cut-off curtain through the formed fracture in a process of dismantling the inner supporting beam; and cutting the bottom-layer main rib to complete dismantling of the inner supporting beam of the foundation pit. According to the dismantling method disclosed by the invention, the end part of the inner supporting beam is chiseled to form the fracture, the vibration waves generated in the dismantling process are blocked through the fracture, and the vibration waves are prevented from transmitting to the water cut-off curtain of the foundation pit, so that influences, on the water cut-off curtain, of the vibration waves are greatly reduced, and water leakage risks are reduced.