Hydraulic cylinder operates under heavy and alternating load，which is very easy to produce fatigue damage. With the expansion of fatigue damage, fatigue strength gradually deteriorates, and even lead to weld cracking and exploding, affecting the supporting characteristics and causing safety hazards. The surface of weld is uneven, and it is difficult to detect by straight beam probe. The transducer is in-stalled on the wedge to detect. According to ray acoustic theory and Snell principle, the propagation path of ultrasonic wave in wedge and workpiece is determined, and the calculation formula of time delay rule of phased array with wedge is obtained. The impulse response of phased array transducer is obtained by superposing spatial impulse response of each element with wedge. The sound pressure on axis of different deflection angles is obtained by simulation, and the characteristics of the acoustic field are analyzed. The phased array experiment on the weld specimen with artificial defects is tested, and defects at different positions can be detected. The hydraulic cylinder weld of ZY9200/25/50D hydraulic support is tested on the spot. The positions and sizes of defects can be directly obtained from the image, and the acoustic field simulation can provide guidance for the testing parameters of ultrasonic phased array.