The invention discloses a combinative drilling tool of a rotary excavating drilling machine, and relates to the field of foundation construction of buildings. The combinative drilling tool of the rotary excavating drilling machine comprises an upper barrel and a lower barrel, wherein the upper barrel is connected with the lower barrel dead against; further, the interior of the upper barrel is communicated with the interior of the lower barrel; the diameter of the lower barrel is less than that of the upper barrel; one end, which is close to the lower barrel, on the side wall of the upper barrel is uniformly provided with multiple drilling teeth along the circumferential direction; a vicinity, which is close to the drilling teeth, on the side wall of the lower barrel is provided with multiple boring mud openings; one end surface, which is far away from the upper barrel, of the lower barrel is provided with a movable opening-closing bottom plate; one end surface, which is far away from the lower barrel, of the upper barrel is also provided with multiple boring mud openings; further, the side wall of the upper barrel is fixedly provided with a spiral steel belt; the spiral steel belt is used for transferring boring mud below the drilling teeth into the boring mud openings of the upper barrel. By using the combinative drilling tool of the rotary excavating drilling machine, the boring perpendicularity can be effectively guaranteed; the boring quality is improved.