Combined drill bit used for cast-in place pile

The utility model provides a combined drill bit used for a cast-in place pile, belonging to the field of building equipment and solving the problems in the prior art that a three-wing drill bit has difficult drilling and big construction difficulty, and a drum drill bit has slower drilling speed. The combined drill bit comprises a center rod, wherein the rear end of the center rod is used for connecting a power source that can lead the center rod to rotate; the middle part of the center rod is fixedly provided with a prismaid-shaped cutter rest taking the center rod as the center; the cutter rest becomes bigger gradually from front to back; and a plurality of drilling cutters 1 are fixedly arranged on the cutter rest. The combined drill bit is characterized in that the front end of the center rod is fixedly provided with a cylindrical drill drum; the size of the outer end surface of the drill drum is less than that of the front end surface of the cutter rest; and a plurality of drilling cutters 2 positioned o