Abstract: In ultrasonic testing of unequal thickness butt joints of low-alloy steel (material 420qD) in a certain project, when the probe was placed on the surface of the long bevel for testing, significant differences in localization and quantification were found when probes at different angles detected the same defect. To address this issue, parameters such as sound velocity, delay, probe front distance, and refraction angle of the probe on the test block were measured, and beam distortion verification was conducted. The research found that in the plate thickness direction, due to differences in grain size from the steel plate surface to the center, ultrasonic wave propagation could also be affected. Therefore, before transverse wave testing of low-alloy steel thick plates and welded joints, the influence of grain size on probe beam performance should be verified, and if necessary, corresponding dedicated comparative test blocks should be fabricated to correct the testing results.