Abstract: Pipelines are susceptible to defects such as corrosion and cracking during long-term service. This study aims to investigate patterns in detecting internal and external surface defects in steel pipelines using the water-coupled ultrasonic detection technique and to enhance the accuracy of pipeline defect detection. To achieve this, a finite element simulation model was developed to replicate the water-coupled ultrasonic detection of thick-walled steel pipes, based on ultrasonic propagation theory. Subsequent studies examined the propagation and sound field distribution of ultrasonic waves in both water media and steel pipelines, as well as the influence of water layer thickness and incident angle on echo signals. To verify the study results, an experimental platform was constructed. The study results are summarized as follows: ① The amplitude of echo signals decreases with increasing water layer thickness, and the ratio of any two echo peaks also declines as the water layer thickness increases. ② The amplitude of echo signals reaches its peak at a water layer thickness of 10 mm. ③ When the water layer thickness exceeds 25 mm, the amplitude of echo signals gradually stabilizes, exhibiting a trend that aligns with the ratio of echo peaks. ④ At a given water layer thickness, the optimal angle of incidence for detecting internal surface cracks is 30°, whereas the optimal angle for detecting external surface cracks is 18°. The research results provide a theoretical basis for water-coupled ultrasonic detection systems, particularly in relation to parameter optimization and the formulation of detection processes.