The invention discloses a combined multifunctional underground continuous wall core tube in a top-down method and a construction method thereof. The core tube comprises a tube body mainly composed of a plurality of reinforced concrete slot segments at intervals. The reinforced concrete slot segments are concrete shear walls. Reinforcement cages of the adjacent reinforced concrete slot segments are connected so that the reinforced concrete slot segments can form the integrated tube body. Door openings are arranged in the intervals between the reinforced concrete slot segments. The core tube integrates the function of supporting piles and posts, the function of a cushion cap, the function of the shear walls and the function of a soil retaining structure. The core tube is of a body structure. Thus, the supporting piles and the posts do not need to be temporarily constructed and detached. The tube body is equivalent to a structure integrally formed through a foundation, the cushion cap and the shear walls. The construction period can be greatly shortened, construction steps are simplified, and construction cost is reduced. Besides, the core tube is of an integrated structure and bears force as a whole, the stress mode is linear stress, and sedimentation of the core tube is greatly reduced.