This paper presents the simulation of temperature in single-track selective laser melting (SLM). The properties of solid metal materials changing with temperature and the phase transition process, as well as the effect of porosity of the metal powder on material properties are considered. The enthalpy-porosity method is adopted to simulate the momentum dissipation during solidification of the melt pool, and the solid-liquid interface is determined. According to the Law of Conservation of Energy, a Parabolic Optical Penetration Depth (POPD) volumetric heat source method is proposed based on the existing cylinder heat source method. The prediction results of the temperature distribution and the melt pool size are compared between the traditional Gaussian surface heat source model and the POPD volumetric heat source model. The results show that the average prediction errors of the molten pool width are 12.18% for the POPD volumetric heat source model, and 7.07% for the traditional Gaussian surface heat source model within the explored laser energy density range. With respect to the width of the molten pool, the prediction accuracy of Gaussian surface heat source is better than that of POPD volumetric heat source mode. When the laser energy density is between 40~100 J/mm3, the average prediction errors of the melt pool depth are 17.00% for the POPD volumetric heat source model, and 38.20% for the traditional Gaussian surface heat source model. With respect to the depth of the molten pool, the prediction accuracy of the POPD volumetric heat source is superior to the Gaussian surface heat source mode. In addition, it is found that the melt pool temperature distribution is much more uniform when considering the melt pool flow effect.