Abstract: This study focuses on continuous casting slab of 45 steel, aiming to explore the influence mechanism of gaseous elements in molten steel on the formation of surface pinhole defects. The influence of gaseous elements on pinhole defect formation was systematically studied by analyzing the characteristics of surface pinholes in continuous casting billets, the relationship between gas content in tundish molten steel and pinhole quantity and depth, and combining with calculations based on gas partial pressure theory. The results indicate that pinholes are unevenly distributed on the cast slab surface and exhibit various morphologies in the depth direction, including channel type, subcutaneous cavity type, subcutaneous chain type, and isolated distribution type. The number of pinholes and their maximum depth are positively correlated with oxygen content. When the mass fraction of total oxygen is no less than 0.003%, the number of pinholes exceeds 100, with depths ranging from 1.5 mm to 4.5 mm. At the mass fraction of hydrogen of 0.000 3%, the number of pinholes reaches 280, showing an upward trend with the increase of hydrogen content but no correlation with the maximum depth. Nitrogen content has no significant correlation with pinhole parameters. Theoretical calculations show that under the conditions of ignoring changes in solubility and setting the bubble escape temperature as the liquidus temperature, the partial pressure reaches 101 325.00 Pa when the volume fraction of active oxygen is 0.004%, 17 225.25 Pa when the volume fraction of hydrogen is 0.001% and 4 053 Pa when the volume fraction of nitrogen is 0.009%. The degree of influence of gaseous elements on bubble formation follows the order of oxygen > hydrogen > nitrogen. After considering segregation, the active oxygen content required to reach a partial pressure of 101 325.00 Pa decreases as segregation proceeds. The research demonstrates that in addition to the original gas content, the segregation and solubility changes of gaseous and carbon elements during the solidification process are also key factors affecting bubble formation.