One of the surface defects on the steel strip surface after the gas-jet wiping employing an air-knife system in the continuous hot-dip galvanizing process is called sag lines or snaky coating. The sag line defect is the oblique patterns such as "W", "V" or "X" on the coated surface. The present paper presents an analysis of the sag line formation and a numerical simulation of sag lines by using the numerical data produced by Large Eddy Simulation (LES) of the three-dimensional compressible turbulent flow field around the air-knife system. In order to simulate the sag line formation, a novel perturbation model has been developed to predict the variation of coating thickness along the transverse direction. The thickness obtained by the proposed model yields very similar results with those obtained by the conventional equation. It is observed that the coating thickness along the transverse direction is affected more by the pressure gradient than the surface shear stress in the stagnation region, while in the far field the shear stress becomes the major factor to determine the thickness. Finally, the three-dimensional coating surface was obtained by the present perturbation model. It was found that the sag line formation is determined by the combination of the instantaneous coating thickness distribution along the transverse direction near the stagnation line and the feed speed of the steel strip. The computed mean distance between the crests and the shape of the simulated sag show relatively good resemblance with the real sag lines on the strip surface.