The rapid melting and solidification of metal powder in selective laser melting process will produce a large thermal gradient, and the dynamic change of temperature gradient will affect the microstructure and mechanical properties of different powder forming parts. Based on finite element analysis software, three-dimensional transient temperature field of selective laser melting forming process was simulated by using moving Gaussian heat source (Gauss) loading. The different properties, temperature changes and solidification cooling laws of Hastelloy X and Ti6Al4V alloys were studied. At the same time, Hastelloy X and Ti6Al4V alloy samples were prepared by selective laser melting process, and their microstructure and mechanical properties were characterized. The results show that the transient peak temperatures of Hastelloy X and Ti6Al4V alloys reach 2 777.37 ℃ and 3 340.88 ℃, and the average cooling rates are 3.93×106 K/s and 5.11×106 K/s, respectively. In the micro-state, grains of Hastelloy X alloy are arranged in feather-like lamination and cross-arrangement, while needle martensite α′ phases are distributed in Ti6Al4V alloy to form a net basket structure. Compared with 728.08 MPa and 338.91 MPa of tensile strength and yield strength of Hastelloy X alloy, the tensile strength and yield strength of Ti6Al4V alloy with "fine grain strengthening" effect at high cooling rate reach 1 134.05 MPa and 1 055.83 MPa, which are much higher than that of Hastelloy X alloy, but its elongation is lower than that of Hastelloy X alloy, which is generally characterized by high strength and low plasticity.