Abstract: The effect of sample thickness on charpy pendulum impact energy absorption of U165 and Q275 structural steels at room temperature was studied by impact test and scanning electron microscopy analysis. The results show that there was no sudden drop of impact load in relatively thin samples, but the impact load sudden drop of Q275 steel with 20 mm sample thickness was more than 80%; when the thickness of the sample was less than 10 mm, the impact absorption energy changed linearly with the increase of the thickness of the sample, but the proportion coefficient of the impact absorption energy changed with the thickness of different materials was significantly different; when the sample thickness was greater than 10 mm, the impact absorption energy deviated from the linear law with the increase of thickness; in addition, with the increase of thickness, the increment of impact absorption energy and fracture morphology of different materials were significantly different, which was attributed to the different impact toughness and impact fracture mechanism of materials. For the material whose thickness did not meet the standard sample, the impact absorption energy of the standard sample could be obtained by testing the non-standard samples with different thicknesses and then calculating by fitting formula.