The invention relates to an ultrasonic phased array detecting and monitoring method, and belongs to the metal welding nondestructive detection field. The method is characterized in that an ultrasonic phased array probe moves at a uniform speed in a direction paralleling to a weld direction, an image and a waveform displaying the echo signal in the weld of a test piece in a phased array ultrasonic instrument display are simultaneously observed while moving the probe, and whether the thin plate weld has a known defect or not can be judged through comparing with a defective etch image and a defective etch signal which are previously stored. The image formed by the ultrasonic echo signal in the thin plate weld can be seen in real time in the detection process, and the image comprises the information comprising the position, the length and the like of the defect, so compared with methods for simply judging the probe quality through the waveform, the ultrasonic phased array detecting and monitoring method has the advantages of accuracy, misjudgment reduction, and improvement of the accuracy and the working efficiency.