Abstract: PAUT (phased array ultrasonic testing) technology for pipe seat fillet weld was not popularized yet, with probe design being the primary challenge. Currently, many scholars have conducted research on PAUT probes for small-diameter thin-walled pipe seats in industries such as nuclear power. However, in oil and gas process pipelines, small-diameter thick-walled pipe seats are used for fillet weld. Unlike other industries, PAUT probe is significantly affected by the detection space, curvature, and focal depth. This paper studied the various parameters of the probe through theoretical and simulation analysis, and for the first time, designed a PAUT probe suitable for the inspection of fillet welds in small-diameter thick-walled pipe seats. This not only provided theoretical support for the design of specialized probes, but also laid the foundation for the application of PAUT in the corner welds of oil and gas pipeline seats.