Abstract: In order to understand the role of laser quenching in composite surface treatment of laser quenching—plasma nitriding of 40Cr, the experiment was designed by using response surface methodology, and the tests were carried out. The relationship between the surface hardness, phases, modified layer depth and the parameters of laser quenching was analyzed. The results indicate that the response surface methodology can be utilized to establish mathematical models correlating the parameters of composite surface treatment processes with the response variables. The order of influence of various factors on the surface hardness is as follows: sample thickness > power > scanning speed, while the order of influence on the depth of the modified layer is as follows: power > scanning speed > sample thickness. During the composite treatment, selecting a lower laser scanning speed, a higher scanning power, and a thinner sample a thickness for the laser quenching process can achieve a deeper modified layer depth, but it can also result in a coarse grain layer, reducing surface hardness. Laser quenching can regulate the phase composition of the surface modified layer.