Under the “double carbon” goal, steel structures are widely used in a variety of large-scale industrial fields. However, steel structures are susceptible to various weld defects due to production process, environment and other factors during connection welding. These defects reduce the stability and service life of the steel structure. Timely detection of defects is important for damage assessment and repair. Aiming at the existing limitations of detecting weld defects in steel structures, it is difficult to balance the detection resolution and detection range. A sixteen-channel high-resolution piezoelectric ultrasonic transducer linear array is designed. Firstly, the structure of the piezoelectric ultrasonic transducer linear array is determined through the optimization of the theoretical model and the device is fabricated. Secondly, in order to test the performance of the developed line array probe, the electrical impedance and ultrasonic performance are tested. The test results show that the deviation between the theoretical design resonant frequency and the experimental test results is less than 5%, and the average -6 dB relative bandwidth of each array element is 79.33%. A broadband linear array probe is successfully designed and developed through the design of acoustic impedance matching layers and wedges. Finally, the linear array probe was used to detect and analyze three types of hole defects with internal diameters of 2 000, 1 000 and 500 μm respectively in a 25 mm thick standard welded test block. The results show that all can be effectively identified and the detection resolution is better than 500 μm, providing technical support for the accurate detection and early warning of tiny defects in thick steel plate welds.