We employ molecular dynamic simulation to investigate metabasin dynamics for supercooled polymer melt.We find that,in a small system,the α-relaxation process is composed of a few crossing events that the monomers hops from one metabasin to another.Each crossing event is very rapid and involves a democratic movement of many particles,whereas such collective motion is not string-like.Evaluation on the contributions of metabasin exploration and democratic movement shows that the structural relaxation is mostly goveed by the latter.Our calculated results show that the metabasin-metabasin transitions are not the main reason of spatially dynamical heterogeneity.It is different from the binary Lennard-Jones mixture model in which the metabasin-metabasin transitions are relevant for the spatially dynamical heterogeneity.