The invention belongs to the technical field of liquid propellant storage tanks, and particularly relates to a butt flange strengthening structure. The technical problem that the bearing capability of an existing butt flange welding line is poor is solved. According to the technical scheme, a strengthening ring is additionally arranged on the inner surface of a butt flange and the inner surface of a storage tank bottom, and a center axis of the strengthening ring coincides with a center axis of the butt flange; the upper surface of the strengthening ring is attached to the inner surface of the butt flange and the inner surface of the tank bottom; and the strengthening ring is connected with the butt flange and the tank bottom through a strengthening ring outer circumferential weld and a strengthening ring inner circumferential weld at a strengthening ring outer sharp point and a strengthening ring inner sharp point. The butt flange strengthening structure has the technical beneficial effects that the stress at a welding line of the flange is obviously reduced, and safety of finished storage tanks is obviously improved through the simple strengthening structure and the mature welding technology