Temperature field is the key factor affecting manufacturing quality in laser welding. For a nail head shape of weld cross-section in the partial penetration laser lap welding, a hybrid heat source model combined with a circular disk source with a Gaussian distribution on the top surface and a linear increasing column source along the depth of weld is proposed, by analyzing the absorption mechanism of the welding material. The effect of the contact thermal resistance between the upper sheet and bottom sheet is considered in the model. The comparison of experimental and simulated results shows that the geometry of weld cross sections is in good agreement. In addition, the relationship is investigated between the thermal cycles and microstructure as well as the micro-hardness. Besides the heating and cooling rate, the peak temperature also has an important influence on mechanical properties of the weld. The performance of the weld is similar when thermal cycles are consistent. The numerical model can help to study the temperature field of the laser deep penetration welding and choose appropriate laser parameters.