The invention relates to an ultrasonic phased array detection technology, and aims at providing a method and a device for detecting the welding seam of a high-pressure hydrogen storage vessel adapter. The device includes an ultrasonic detector and a curved-surface wafer ultrasonic phased array probe, wherein a curved-surface wafer is arranged on the probe surface; the device also includes a probe mounting rack, a hollow screw, a screw base and a handwheel; the probe mounting rack shelf is fixed to the top end of the hollow screw; the center of the screw base is provided with a threaded hole; the outer surface of the hollow screw is provided with matched thread; the screw base sleeves the hollow screw; the center of the hand wheel is provided with a through hole; the handwheel fixedly sleeves at the end of the hollow screw; a probe line passes through the hollow screw and is used for connecting the ultrasonic detector and the phased array probe. The method can realize defect detection of the vessel adapter forgings and hemispherical seal head butt welding seam and heat affected zone; the detection is simple for operation, and method and the device can effectively detect all volume defects and longitudinal defects, so that the detection accuracy and reliability are obviously improved.