Abstract: In order to effectively detect small thin-walled tubes in nuclear power plants, the water immersion focused S-wave method and P-wave method were introduced. The technological parameters of the probes were studied and determined, and the comparative detection was carried out on the sample tubes with local wear, annular wear, axial cracks, circumferential crack,through holes and other defects. The results show that the two ultrasonic detection methods had their own advantages and disadvantages in the detection and quantification of different defects. Among them, the water immersion focused P-wave method had a strong detection ability for the defects such as wear, swelling and through hole, and the quantification of defects were accurate. Water immersion focused shear wave method was strong in crack detection, but difficult to quantify. The combination and mutual verification of the two methods can greatly improve the ability of defect detection.