Molecular dynamics was employed to study the effects of the size of helium cluster and the helium content on the elastic properties and the volume change of the material under different pressures. We tried to analyse the cause that induced volume change of titanium and the instability of the system. With the increasing of helium content, the cohesive energy reduces, the external volume of the material expands, and the materials becomes unstable. When the size of He cluster is small, with the helium bubble growing, the cohesive energy rises, a volume contraction of the material occurs, and the system tends to keep stable. When the number of helium atoms in helium bubble is over 74, it is difficult for the helium bubble to grow.