A combined drilling tool comprises a hollow rod body, a plurality of tool bodies, an expansion cutter arbor, a lower blade, an oil cylinder and an upper blade, wherein the tool bodies are fixed on the blades; the lower blade has a peripheral surface which is uniformly distributed and fixed at the lower part of the hollow rod body in a horizontal radial manner; the expansion cutter arbor is arranged below the lower blade, and arranged symmetrically along the hollow rod body in the radial direction; one end of the expansion cutter arbor is hinged to a pin seat of the hollow rod body; a connecting rod penetrates a straight groove formed on the wall of the hollow rod body; one end of the connecting rod is hinged with the middle of the expansion cutter arbor; and the other end of the connecting rod is hinged to a connecting fork at the end of the piston rod of the oil cylinder. Besides the common straight hole drilling function, the combined drilling tool for expanding a bottom, which adopts a combined-type drilling structure and further has the advantage that when bottom expanding or broaching operation is required, the expansion cutter arbor can be opened by operating the oil cylinder, so as to enlarge the sectional area of the pile end and the pile body of a bearing pile. The drilling tool can be used for building various bottom expanding piles and gourd poles, and has the obvious characteristics of convenience in construction, simplicity in operation, short construction period and the like.