Abstract: The conventional online automatic detection of hydrogen storage steel cylinders typically utilizes the water-immersion ultrasonic testing technology with point or line focusing method. However, the point focusing method has slow detection speed, and the line focusing method has low sensitivity and resolution. This paper proposes a phased array detection technology utilizing a concave acoustic lens that combined water-immersion to focus ultrasound in the radial direction of the steel cylinder. The radiated sound field of the phased array probe with a concave acoustic lens was simulated and analyzed by CIVA. The phased array probe parameters were optimized in accordance with the simulation results. Based on this, a five-axis linkage ultrasonic phased array water-immersion C-scan automatic detection system for hydrogen storage steel cylinders was developed. The system quickly planed the path of ultrasonic testing and reconstructed surface through electronic scanning technology of phased array. The system was tested using a hydrogen storage steel cylinder with artificial defects. The results showed that the proposed system, combining the advantages of the point focusing and line focusing methods, can achieve automatic online ultrasonic detection of axial and circumferential defects in hydrogen storage steel cylinders.