The shielding effect of capsule was studied during the processes of Hot Isostatic Pressing (HIP) by numerical simulation and HIP experiment of aluminum alloy powder. In addition, the experimental equations about the shielding effect of capsule were established. The results show that the shielding effect of capsule depends on geometry size of capsule, wall thickness and yield strength. The reduced pressure due to the shielding effect of capsule presented an approximate U-shaped distribution. The shielding effect of capsule has an enormous impact on grains rearrangement stage. The thickness of capsules defer the start time of powder densification in an early stage of HIP and retard the powder densification rate, which leads to the reduction in the final density of components.