Abstract: In order to solve the problem of fatigue crack detection in U rib fillet weld of steel box girder of bridge, the fatigue crack characteristics of U rib fillet weld were studied. By analyzing the causes of fatigue cracks and the characteristics of various non-destructive testing methods, it is found that the ultrasonic phased array testing is the most suitable one. In order to verify the practicability of ultrasonic phased array in detecting fatigue cracks of U-rib fillet weld, the section model of U-rib fillet weld was established through simulation software and the acoustic beam coverage was also simulated. On this basis, a set of U-rib fillet weld test blocks with artificial defects were made for indoor verification, and finally applied to the field practice of a bridge.