By solving the time-dependent Schrödinger equation for the interaction of the intense laser field with the two-dimensional model of H ion cluster, it is found that the high-order harmonic plateau produced by H ion cluster is wider than that generated by a single H atom. The interaction between intense laser field and cluster is decomposed into three processes: internal ionization, classical motion under the action of external field and Coulomb field of the cluster ions, and recombination. After internal ionization, the particle is deemed classical and its motion follows Newton’s equation of motion. By studying the classical trajectory of electron and the variation of kinetic and potential energy with time, it is observed that during the electron’s returning, the additional kinetic energy is required as a result of the reduction in potential energy. Furthermore, the correlation between return energy and return time obtained from the classical model is in good agreement with that obtained from time-dependent Schrödinger equation. In this study, the cutoff energy of high-order harmonic generated by clusters is compared with that of a single atom, indicating that the extension of the platform of high-order harmonic by clusters is primarily caused by the Coulomb effect of other ions surrounding the parent nucleus. Additionally, the influence of ion spacing on the cutoff energy of high-order harmonic is also investigated, and a possible relationship between the cut-off energy of high harmonic and the cluster expansion is established.