Inconel 718 is commonly used in aerospace, automotive and medical equipment, but it is a typical difficult-to-machine material due to its high strength and low thermal conductivity, resulting in large cutting forces and high cutting temperatures. In this paper, the true stress–strain curve of Inconel 718 at room temperature was obtained by the split Hopkinson pressure bar experiment, and the thermal softening rate of the material at different temperatures was obtained by the high-temperature hardness experiment. The laser thermal conductivity experiment was used to obtain the specific heat capacity and thermal conductivity of Inconel 718 at different temperatures, and the actual deformation temperatures at different strains were calculated by combining the true stress–strain curves. The decoupling of strain and temperature is achieved by using the thermal softening rate to correct the true stress–strain curve in the variable temperature state to the stress–strain curve in the isothermal state. The above experimental results were fitted based on the Johnson–Cook and Power–Law constitutive models, and the results show that the fitting accuracy of the Power–Low constitutive model is higher at low strain rates, while that of the Johnson–Cook constitutive model is higher at high strain rates. Finally, the response mechanism of the mechanical properties of Inconel 718 material under the action of strain, strain rate and temperature alone are explored through finite element simulation, and it is found that the strain has the greatest influence on the stress, followed by the temperature, and the strain rate has basically no influence on the stress.