The invention discloses a method for quantitatively and qualitatively detecting root defects when the butt-jointed seam of the prior thick-wall (the thickness is more than 40mm) big-diameter tube can only be detected from a single side by adopting one-vertical one-horizontal high-frequency and narrow-pulse double probes based on a digital ultrasonic defect detector. For a thin-wall tube or tube on which the two-side detection is allowed, the digital ultrasonic wave based defect detector also can be used in reference. The horizontal wave inclined probe is moved, a main acoustic beam can generate the strongest diffracted waves when hitting the top part of the root defects such as crack, incomplete fusion, lack of fusion and the like or root shape defect aberration serious parts such as welding beading, inward depression, misaligned edges and the like, and the vertical wave straight probe on the welding seam receives the strongest diffracted waves; the horizontal wave inclined probe can read out the depths of top points of the defects, and the thickness of the wall of parent metal on the side of the inclined probe can be measured by moving the vertical wave straight probe; therefore, the quantitation on six types of typical root defects is realized; and simultaneously, by combining the quantitative data such as heights and horizontal positions of the defects, which is measured by a one-vertical one-horizontal double probe strongest diffracted wave method, with the characteristics of the diffracted waves generated when the horizontal wave inclined probe rotates to detect, the qualitation on the six types of typical root defects is completed, thereby solving the problem that the root defects are difficultly judged when the butt-jointed seam of the thick-wall big-diameter tube can only be detected from the single side.