Field-ion microscopy measurements show a large value of the diffusion prefactor for the Pt7 cluster, exceeding those of small Ptn(n≤4) clusters by two orders of magnitude. In the present paper, with long-time (∼0.2 μs) simulations and tracing of interstitial atoms on the surface, a linear relation between the mean square displacement of the cluster's mass-center and the simulation time is obtained. The activation energy and prefactor for two-dimensional atomic clusters Ptn(1≤n≤7) on the Pt(1 1 1) surface are determined by molecular dynamics. The present simulations can interpret the experimental data reasonably. The large prefactor of the Pt7 cluster could be attributed to a large number of nonequivalent diffusion processes with similar activation energies on the soft Pt surface.