Abstract: Test samples with typical defect signal of ultrasonic testing of nuclear zirconium alloy cladding tube were selected, including the same position longitudinal and horizontal defect, single vertical defect and single lateral defect with different signal amplitudes of 20%~25%, 25%~30%, 30%~35% and ≥ 36%, respectively. The test samples were investigated by macroscopic detection and metallographic method, and the relationship between the type, depth and position of defects in cladding tube and the type and amplitude of ultrasonic detection signals was analyzed. The results show that defects of depth of 10 μm or above generally cause abnormalities of ultrasonic detection signal, and the relationship between the depth of defect and the amplitude of the ultrasonic signal is basically linear, that is, the higher the amplitude, the deeper the depth of the defect. The cracks of the inner surface, outer surface and inside of cladding tube will cause the abnormalities of ultrasonic longitudinal, horizontal, as well as the single vertical detection signals, while the pits on the inner surface and outer surface of the cladding tube will cause the abnormalities of ultrasonic lateral detection signal. The grassy type of signal is not directly related to the grain structure, surface roughness and defect of cladding tube.