Abstract: Large-diameter steel bars are essential structural materials in engineering applications, and the existence of their internal defects will directly affect the service safety of components. To address the current challenges of low inspection efficiency and insufficient automation in ultrasonic testing of large-diameter steel bars, a fully water immersion phased array ultrasonic testing (PAUT) automatic system has been developed. The system integrates electronic focusing of phased array technology with C-scan imaging and employs a spiral scanning method to achieve full-volume coverage for steel bars with diameters ranging from ϕ100 mm to ϕ355 mm. Full automation of the inspection process is realized through integration of automatic loading and unloading, intelligent transfer units, and a programmable logic controller (PLC) system. Using artificial defects reference bar specimens containing flat-bottom holes and transverse holes, the tests on key indicators such as signal-to-noise ratio, stability and blind zone are conducted according to YB/T 4082-2020 standard. The performance evaluations are compared with multi-channel fully water immersion ultrasonic systems and through-type phased array systems.The results show that the developed system can accurately detect the specified equivalent artificial defects. Specifically, it achieves a signal-to-noise ratio (SNR) not less than 12 dB, the detection sensitivity of the same channel with of a fluctuation not more than 1 dB over a continuous 4 h period, and an end blind detection zone not greater than 10 mm. All these three indicators are higher than the requirements of YB/T 4082-2020 standard. Under the premise of maintaining equivalent inspection sensitivity, its inspection efficiency is significantly higher than that of the multi-channel full water immersion ultrasonic testing method. Moreover, it effectively solves the technical limitation that the existing through-type phased array systems are difficult to inspect large-diameter bars with diameter over ϕ220 mm. The system has been successfully deployed in an industrial production line, which has verified its engineering applicability and reliability, providing an effective non-destructive testing solution for quality assurance of large-diameter steel bars.