Abstract: The problem of corrosion inspection of the oil and gas storage tank bottom in service has not been effectively resolved due to the influence of factors such as test conditions and the contents.In this article,the Lamb waves of mainly A0 and S0 modes have been inspired by using the variable angle probe based on ultrasonic Lamb wave mode and dispersion property.The experiment was carried out on the reference plate of tank bottom magnetic flux leakage testing to study the influences of wave structure to the Lamb wave detection and the relationship between the cross-sectional area of defects and the echo signals amplitudes.The optimal angle of excitation is determined for a certain thickness of the steel plate and the experimental results are analyzed from the acoustic vibration displacement distribution and energy density distribution.The study has shown that under certain conditions of the operating frequency,the wave structure is an important factor of affecting Lamb wave detection.Besides,defect echo signal amplitude becomes larger with increasing cross-sectional area .The results show that different modes for the same types of defects have different sensitivity to changes in cross-sectional area and the depth of detects and the degree of sensitivity to changes in cross-sectional area and the depth of detects is different for the same types of defect mode.