Considering the Coulomb explosion induced by the interaction of a deuterium cluster target with an ultra-intensie femtosecond laser, the mechanism which generates energetic deuterium nuclei for the fusion has been analyzed．The formulas for expansions of deuterium ion clusters, which are driven by Coulomb explosion, are proposed; and hence the kinetic energies of deuterium nuclei and the expansion time of deuterium ion clusters have been estimated．