The invention relates to a portable phased array ultrasonic quantitative detection device and a portable phased array ultrasonic quantitative detection method for a planar lap joint short welded joint, and belongs to the field of welded joint quality nondestructive detection. An industrial computer is internally provided with a data acquisition card and is connected with a phase array ultrasonic acquisition card and a power supply control module separately through data lines; the power supply control module and the phased array ultrasonic acquisition card are connected with a scanner separately to perform data acquisition and transmission on signals transmitted and received by a phased array ultrasonic probe in the scanner; the power supply control module performs driving and start-stop control on a stepping motor in the scanner; and the scanner is internally provided with the phased array ultrasonic probe and the stepping motor, and is an ultrasonic signal transmitting/receiving /scanning execution device. The device and the method have the advantages that high-precision fully-automatic quantitative detection on defects in the welded joint is realized by a low-cost handheld structure, and numerical value and image feedback are formed through computer fitting.