For many years, rivet joint technology has been applied in the automotive and aerospace industry. Recently, it began to apply laser welding technology to lap joints instead of rivet joining. Laser spot welding has some potential advantages including time saving, cost reduction, material saving and weight reducing. A lap joint of aluminum alloy LY12 with different plate thickness, namely 2mm and 1mm, was spot-welded by CO2 laser. For the welding, laser power in pulse form with ramping-up and cooling-down shape was used, and pure helium gas served as shielding gas to fill around welding area. In this study transient three-dimensional non-linear finite element modeling was used to analyze heat flow and residual stress of the laser spot welding of aluminum alloy LY12. In modeling the temperature dependence of material properties, influence of contact surfaces are taken into account. To analyze, Gaussian distributed heat source model and thermo-elasto-plastic behavior were applied. Weld dimensions and residual stress at the weld surface were calculated numerically and compared with the experimental results.