The surface absorption coefficient of S304L stainless steel during Nd:YAG pulse laser welding process was derived by using the inverse engineering technique. The thermal-elastic-plastic finite element method is employed to calculate the temperature distribution, melting pool depth and shape with specific heat source. The derived melting pool depths and diameters with different values of absorption coefficient were compared with experimental results. In this study, the least square error method is used to obtain the equivalent absorption coefficient. The results show that the equivalent absorption coefficient of S304L stainless steel is in the range of 0.1~0.25 for low power intensity Nd:YAG impulse laser welding.