The ice surface provides two-dimensional reaction fields for bimolecular collisions in interstellar space. As H2O molecules on the surface are typically exposed to cosmic rays, H2O in the excited state is easily dissociated into H + OH, where a H atom is released from the surface to the gas phase. In the present study, the reaction dynamics of small-sized water clusters on the triplet-state potential energy (T1) surface, following the vertical electronic excitation from the ground state (S0), were investigated using direct ab initio molecular dynamics to provide insights into the generation mechanism of H atoms from an irradiated ice surface. In all clusters, that is, (H2O)n (n = 2-6), the H atom was directly dissociated from one of the H2O molecules in the clusters (direct dissociation), whereas the OH radical remained in the cluster. In the branched form of H2O tetramer (n = 4) and the book form of H2O hexamer (n = 6), the dissociated hydrogen atom (H') collided with the neighboring H2O molecule, and the exchange of H atoms occurred as H' + H2O → H'-H2O (collision) → H'OH + H (hydrogen exchange). The translational energy of the exchanged H atom decreases significantly (by approximately 50%) because the kinetic energy of the H' atom is efficiently transferred to the vibrational modes of the cluster during the H-exchange reaction. The mechanism of H atom dissociation is discussed based on theoretical results.