The invention relates to a long auger drilling and bottom expanding device and an expanded anchor rod construction method. The long auger drilling and bottom expanding device comprises an auger drilling rod with an auger blade and a front guide drill bit. The upper portion of the auger drilling rod is connected with a power box of a pile machine through a rotary connector, a slurry inlet is formed in the rotary connector, motors are arranged on the two sides of the upper portion of the power box, a broaching oil cylinder support and a broaching oil cylinder are arranged in the middle of the upper portion of the power box, a connecting lug is arranged in the middle of the upper portion of the oil cylinder support, the broaching oil cylinder is hinged to the broaching oil cylinder support through the connecting lug, the lower end of a piston rod of the broaching oil cylinder is connected with the top of a broaching connecting rod through a transfer connector, the lower portion of the broaching connecting rod is arranged in an axial central through hole of the auger drilling rod, a rotary broaching device is arranged between the front guide drill bit and the auger drilling rod, the lower end of the broaching connecting rod is connected with the rotary broaching device, and the lower portion of the rotary broaching device is connected with the front guide drill bit. According to the long auger drilling and bottom expanding device and the expanded anchor rod construction method, noise, vibration and slurry are avoided in the construction process, an anchoring body is high in strength, good in quality, low in cost, high in working efficiency and good in environmental friendliness, and the uplift bearing capability of an anchor rod is high.