Abstract: The hot wall hydrogenation reactor operates under harsh conditions such as high temperature, high pressure, and hydrogen corrosion for a long time, and defect detection of the inner wall cladding layer is crucial for equipment safety. A multi-mode fully focused imaging detection method based on mirror echo estimation is proposed to address the technical difficulties of ultrasonic testing on the outer side of the hydrogenation reactor weld layer. Firstly, a mirror echo estimator is used for simulation analysis to obtain the sensitivity of the multi-mode fully focused imaging area, and to determine the ultrasonic path sensitive to defects in the weld overlay; Then, obtain the fully focused imaging features of crack defects under different paths; Finally, the multi-mode fully focused imaging method is used to detect and locate crack defects of different sizes on the surface of the test block weld layer. The simulation and experimental results have verified the reliability of this method in detecting inner wall cracks and locating defects accurately, providing a more economical and efficient detection solution for the industry.