Inconel 718 pre-alloy powder was prepared using the electrode induction melting gas atomization (EIGA) and vacuum induction melting gas atomization (VIGA) methods. Inconel 718 alloy was prepared using hot isostatic pressing (HIP). The effects of milling process, powder particle size, and capsule structure on the microstructure and mechanical properties of Inconel 718 alloy were studied. The results show that EIGA powder has better sphericity, and the mechanical properties of the forming alloy are better than those of VIGA powder. Considering the mechanical properties, powder yield and other factors, the powder with particle size range of 15–106 μm was selected as the hot isostatic pressing powder. The wall thickness of the capsule has a shielding effect on the effective stress acting on the powder. The change of cavity volume has little effect on the densification process. Long term vibration during the powder filling process can lead to particle size segregation, and it is necessary to control the vibration frequency and time during the powder filling process.