The utility model discloses an integrated in -pile support structure's of reactor pressure vessel low head changeover portion, wherein, changeover portion portion inner wall sets up water flat groove, water flat groove comprises 4 planes, and the plane includes horizontal plane and perpendicular, compared with the prior art, still including the welding at the inboard in -pile supporting part of changeover portion portion, the in -pile supporting part is the rectangle piece, the rectangle piece is at least including 2 sides with perpendicular size unanimity. Weld the stage of popularization that comprehensive electric smelting increases the material to the tradition, the utility model discloses, that adopts sets up water flat groove in changeover portion portion, and the in -pile supporting part that further uses the rectangle again embeds water flat groove, and both are through welded connection, such connected mode, and welding seam is the L type, and during holistic in -pile supporting part atress, it is vertical decurrent that gravity is made a concerted effort.