The growth of helium cluster in titanium crystals at room temperature has been simulated detailedly by means of molecular dynamics. The mechanism of helium cluster growth in metal materials has been investigated from the viewpoint of energy. It is found that, as the helium cluster grows, dislocation loops are formed around the cluster and the binding energy of each additional helium atom to these clusters tends to decrease. However, at certain point in the growth process, the defects escape rapidly and the binding energy increases, helping the further growth of the helium cluster. As more helium atoms are introduced, the shape of the helium cluster is transformed gradually to a prismatic morphology from the irregular morphology and its thickness is always observed to be about 1.2 nm, without any obvious variation with degree of helium filling.