Traditional and classical depth profile analysis is very time consuming and difficult to analyze irregular coating sample. In the present work, under optimal parameters, two irregular unknown coating samples were analyzed by laser-induced breakdown spectroscopy. Depth profile analysis curve indicates that zinc diffusion process is adopted for this two unknown samples. Zn and Fe are major elements, and small amount of Cr is used to passivate metal surface. Under the interface, the substrate is medium-low steel alloy, a set of certified reference material for medium-low steel is used to establish calibration curve, and chemical composition of substrates and the concentration variation for Cr and Mo from surface to interface were investigated in detail.