Thermal cycle greatly influences the microstructures and mechanical properties of laser welded joint. Taking account of the heating effect of plasma on the workpiece and the absorption of heat in the keyhole, a mathematical model, consisted of a point heat source and a line heat source which weakens with the depth, is put forward, and a delamination method is applied for the analysis. The model is used to calculate the thermal field of laser welding of mild steel. The results are compared with that of experiment. The calculation results coincide well with the experiment results.