This article presents the influence of the process parameters in laser transmission welding for plastics on the residual stress in the welded part. The contour welding process is modeled by means of finite element (FE) simulation. In this process, the weld seam is only partially heated, i.e., only part of it melts. The calculations are performed using a material model that describes the time-dependent temperature and stress development in a plate geometry, making allowance for the material's asymmetric compressive-tensile behavior. Experimental data were measured under different load cases to present the time-dependent material behavior, and then implemented in numerical terms by formulating the necessary constitutive equations. The calculations to simulate the influence of process parameters on the residual stress behavior were performed using a finite element model that was developed. The simulation covers the entire welding process, including the heating and cooling stages.