The invention provides a combined drilling tool, which comprises a drill stem, a drill bit and a drill bit joint. The drill bit is connected with the drill stem through the drill bit joint; the drill stem is connected by drill stem sections; each drill stem section comprises an inner stem section and a hollow outer stem section; the inner stem section can be rotatably fixed in the outer stem section; the inner stem sections of the drill stem sections are connected to form an inner stem of the drill stem; the outer stem sections are connected to form an outer stem of the drill stem; the inner stem is connected with an output shaft of power machinery; and the outer stem is fixedly connected with a shell of the power machinery. For the provided combined drilling tool, the length of the drillstem can be optionally adjusted to meet the demands of different drilling depths; the outer stem is fixedly connected with the shell of the power machinery immovably; and the inner stem is connected with the output shaft of the power machinery and can rotate inside the outer stem at a high speed but does not have friction with the hole wall. Therefore, the drill bit rotates at a high speed on thepremise of no rotary friction between the drill stem and the hole wall, and the drilling speed and efficiency are greatly improved.