An assembled drilling tool has a drilling bit (2) connected with a drilling stem (1) through a drilling bit joint (3). The drilling stem is connected by drilling stem sections (11). Each drilling stem section includes an inner stem section (111) and a hollow outer stem section (112). The inner stem section is rotatably fixed in the outer stem section. The inner stem sections are connected together to form inner stem (11′) of the drilling stem, and the outer stem sections are connected together to form outer stem (12′) of the drilling stem. The inner stem is connected with an output shaft of a power machine, and the outer stem is fixedly connected with the shell of the power machine. The length of the drilling stem of the assembled drilling tool can be arbitrarily adjusted, and the drilling bit can be rotated with high speed without friction generated between the drilling stem and the hole wall, thereby improving the speed and the efficiency of hole drilling.