Abstract: In order to reveal the plating mechanism that the steel substrate pre-treated by laser quenching could refine the grain of the Cr-plating coating interface, theoretical analysis and experimental verification were carried out. Both sides of the interface between laser discrete pre-quenching substrate and Cr-coating were prepared by chemical etching method: Cr-plating coating interface and substrate interface. The grain morphology of Cr-plating coating interface was researched by scanning electron microscope and the roughness of substrate interface was measured by laser roughness instrument. Based on plating theory, the analytical model of the relationship between the grain size and the roughness was developed with the help of the over-potential. The test and theoretical results indicated that the roughness was proportional to the grain size. The results show that the plating mechanism of grain refinement of Cr-plating coating by laser pretreatment substrate is that the smaller roughness could increase the over potential, which is obtained by laser pre-quenching substrate. The increase of over-potential reduces the grain size. The result is helpful for further analysis of laser pre-processing substrate to improve the interfacial bonding strength.