In this study, we examine the relationship between the numerical simulation and experimental validation of laser welding for dissimilar aluminum–copper joints, with a focus on temperature distribution, molten pool evolution, and weld bead geometry. A finite-volume-based thermal model was established using a 1000 W laser power, a scanning speed of 275 mm/s, and a spot radius of 0.06 mm. At the same time, corresponding experiments were conducted under identical conditions to capture the cross-sectional features of the welded joint. The simulation results showed strong agreement with experimental measurements, with a mean absolute error of less than 10% across most evaluated parameters, including penetration depth and bead width. However, a localized deviation of approximately 22% was observed in the heat-affected zone boundary, which can be attributed to simplifications in thermal boundary conditions, the use of temperature-dependent material properties, and experimental measurement uncertainties. Despite these discrepancies, the developed model successfully captured the dominant heat transfer and melting mechanisms in aluminum–copper laser welding, preserving both accuracy and fidelity to the trend. The validation demonstrates that simulation provides a cost-effective and reliable predictive tool, reducing the need for extensive experimental trials while enabling process optimization in joining dissimilar metals. The findings highlight the potential of simulation-driven welding design for accelerating parameter selection, minimizing resource consumption, and improving manufacturing efficiency. Future refinements, including the incorporation of temperature-dependent thermophysical properties, calibrated absorptivity, and melt-pool convection physics, are anticipated to enhance predictive accuracy and extend the model’s applicability to more complex joint geometries and variable process conditions.