Bridge construction rotary drilling rig

The invention discloses a bridge construction rotary drilling rig which structurally comprises a mast assembly, a walking chassis, an amplitude-variable frame, a drill pipe and a drilling tool, wherein the amplitude-variable frame is fixedly mounted on the walking chassis; a hydraulic system controls the amplitude-variable frame and the mast assembly; the drill pipe and the drilling tool are arranged on the mast assembly; a power head is arranged at the end, far away from the drilling tool, of the drill pipe; and the drilling tool comprises a drilling disc, a high-speed rotary machine, a rotary shaft, an external bearing and a drill hole. According to the bridge construction rotary drilling rig, during the drilling process, a sealing knockout plate and servo heads arranged in matched rotary shafts are jointly matched with a retaining body and an inner shaft; during the soil unloading process, a rotary motor rotates forward to back out the inner shaft meshed with the rotary motor, during the inner shaft backou