In this study, an L-shaped model was designed to simulate the constraining structure of the laser additive strip sidewall of a valve seat to construct the constrained structure. The basic laser additive simulation model considers the changes in laser absorptivity, latent heat of phase change of the material, and heat exchange between the additive process and the external environment. Results show the existence of a competitive relationship between the laser energy density change and the energy input of the laser in space. When the effect of increasing the laser power and power density on the temperature rise is greater than the effect of increasing the speed and reducing the energy in the space on the temperature drop, the temperature of the additive layer increases. Because of the influence of the substrate structure asymmetry and the heat transfer and dissipation in the laser additive process, the depth of fusion between the additive layer and the bottom surface of the substrate is greater than the depth of fusion with the side surface and the cross section of the additive layer has a stepped-crescent shape. Furthermore, because of the combined effects of the hysteresis of heat conduction and rapid heating and cooling of the laser, with the increase in laser power and scanning speed, the cross-sectional profile of the additive layer gradually switches from a crescent to a platform shape.