Based on the forming principle of Hastelloy X alloy prepared by laser melting (SLM), the DLUX subroutine is written in Fortran language to load Gaussian light source, and finite element analysis software ABAQUS is used to numerically simulate the transient temperature field and cooling rate of finite element model, and the analysis results are verified by experiments. The heat transfer, melting, liquid metal flow and solidification process of powder particles and Gaussian light source during forming were studied. The results show that the microstructure of Hastelloy X alloy presents equiaxed crystal with fish scales in cross section and feathery columnar crystal in vertical section. SLM forming produces a large temperature gradient, which is a non-equilibrium dynamic process with high cooling rate. The average cooling rate is 3.02× 106℃/s. Under the effects of high cooling rate and fine grain strengthening, the tensile strength of vertical and cross sections reach 97% and 89% of that of forging, respectively. The yield strength is far better than that of forging process. The vertical section shows high-strength plastic matching performance, meeting the requirements of industry standards.