As a key feature of a part or assembly, surface structure can effectively improve the surface properties of the workpiece. Laser remelting is a promising method for surface structure formation, but sensitive to ambient gases. Therefore, this paper presents a numerical model that incorporates the contribution of air atmosphere on surface bulge structure formation. For this, the model couples a concentration field to reflect the oxygen mass transfer in the melt pool and introduces a semi-empirical function to describe the contribution of oxygen to the surface tension. Based on the model, it discusses the surface structure formation mechanisms, the heat-mass transfer characteristics, the melt flow patterns, and the surface force transient evolutions. The results reveal that the action of air atmosphere on melt flow manifests itself through two mechanisms, namely, the enhancement of inward thermocapillary forces and the introduction of outward solutocapillary forces. Furthermore, combined with the laser remelting experiments on 304 stainless steels, the established model is validated concerning the surface bulge structure size, the melt pool depth, and the oxygen distribution.