The utility model relates to the technical field of drilled pile construction, in particular to a construction device capable of drilling to the bottom of a stratum with the hard lower portion and the unconsolidated upper portion. The device can solve the problems that during the existing drilling construction process of the stratum with the hard lower portion and the unconsolidated upper portion, difficulty is large, processes are complex, and work periods are long and the like. The device comprises a rotary digging machine and a long protective barrel, wherein the rotary digging machine is provided with a rotary digging barrel, the long protective barrel is located in an upper portion unconsolidated stratum hole, the rotary digging barrel extends into a lower portion hard stratum hole, the lower end of the rotary digging barrel is welded with a tooth-shaped drill, and one side of the upper portion of the rotary digging barrel is provided with a reamer. The structure comprises an installation plate, a square installation hole is formed in the installation plate, an L-type plug-pin is arranged in the installation hole, locating screw holes are formed in the L-type plug-pin in a spaced mode, and a locating screw rod is connected with the installation plate. The device has the advantages of being simple in process, advanced in technology, high in construction efficiency, strong in practicality and the like, has good economic and social benefits, is suitable for drilling and pile pouring of the stratum with the unconsolidated upper portion and the hard bottom portion, and is also suitable for the drilling and the pile pouring of single loose sand and other engineering.