Based on the ANSYS finite element simulation software, a three-dimensional solid finite element model of laser melting RuT300 is established, in which the effects of laser absorption rate, thermophysical parameters of materials and latent heat of phases are all considered. The effect law of preheating temperature on temperature distribution, temperature gradient, cooling rate, and so on is obtained by the analysis of the transient temperature fields of laser fusion under different preheating temperatures. The results show that the maximum temperature at the same position increases with the increase of preheating temperature. The preheating temperature has a more obvious influence on the temperature field at the low part of the sample. With the increase of preheating temperature, the temperature gradient at the same position of the sample decreases. When the depth from the upper surface of the sample is about 2 mm, the temperature gradient of the sample is not obviously influenced by preheating temperature. In addition, the preheating treatment of samples can be used to reduce their cooling rates, and it is found that the cooling rate decreases as preheating temperature increases.