The work aims to improve the depth and uniformity of the hardened layer of 42CrMo steel after laser quenching. The temperature field evolution of 42CrMo steel during laser quenching was analyzed by COMSOL Multiphysics software, and the variation of thermal properties of the material along with temperature was considered. The temperature field distribution of the sample was simulated by setting the laser processing parameters, and the morphology size of the hardened layer was obtained under the martensitic transformation conditions. According to the simulation results, the laser quenching experiment was carried out to 42CrMo steel by fiber-coupled diode laser with continuous wave. The temperature of the sample was measured by a thermocouple thermometer and compared with the simulated temperature history curve. The morphology of the hardened layer at the cross section of the sample was analyzed by optical microscopy and the morphology of the hardened layer obtained by the experiment was compared with the simulation results. The laser quenching process was simulated by changing the spot size under the same laser power density and the geometrical characteristics of hardened layer were analyzed and compared. The temperature history curve of a certain point measured by the experiment was consistent with the simulation result, and the morphology of the hardened layer obtained by the experiment was basically consistent with the simulation results. When the laser power density was constant, the width of the hardened layer increased proportionally with increase of the width of the spot perpendicular to the scanning direction, and the depth increased first and then decreased. The curvature of the point with the same distance from the deepest point in the center of the hardened layer gradually reduced. By optimizing the processing parameters of laser quenching and controlling the heat conduction time and temperature gradient distribution in depth direction of laser quenching, a deeper hardened layer can be obtained without surface melting. Spot size has a great influence on the depth and uniformity of laser hardened layer of 42CrMo steel and a larger spot width can be selected to obtain a more evenly distributed hardened layer.