The invention provides a variable-diameter spiral drill rod which comprises core pipes and screw blades. The variable-diameter spiral drill rod is characterized in that variable-diameter blocks are arranged on the core pipes between the screw blades and in the direction of the screw blades, the variable-diameter blocks continuously or segmentally enlarge the diameters of the core pipes from bottom to top, and in the drilling process, the variable-diameter blocks can gradually compact downwards and outwards soil mass drilled by the screw blades. The invention further provides a method for construction by utilizing the variable-diameter spiral drill rod. The method comprises a step 1) of pre-forming holes without coming out of soil at a pile position; a step 2) of placing a drill rod in a pile hole; a step 3) of performing rotary drill rod drilling, and squeezing the drilled soil mass into a hole wall through the variable-diameter blocks; a step 4) of performing repeated operation to a set depth to form the pile hole; a step 5) of conveying concrete into a drill rod material-conveying pipe while lifting up the drill rod; a step 6) of performing continuous operation until the drill rod is lifted out to from a concrete pile; a step 7) of executing a step (1) of reversely inserting a reinforcement cage in a pile body when the pile is used for a pile foundation; and executing a step (2) of laying a cushion layer on the surface of the pile top when the pile is used for a composite foundation.