An energy model for the structure transformation of pile-ups of grain boundary dislocations (GBD) at the triple-junction of the grain boundary of ultrafine-grain materials was proposed. The energy of the pile-up of the GBD in the system was calculated by the energy model, the critical geometric and mechanical conditions for the structure transformation of head dislocation of the pile-up were analyzed, and the influence of the number density of the dislocations and the angle between Burgers vectors of two decomposed dislocations on the transformation mode of head dislocation was discussed. The results show when the GBD is accumulated at triple junction, the head dislocation of the GBD is decomposed into two Burgers vectors of these dislocations unless the angle between the two vectors is less than 90°, and the increase of applied external stress can reduce the energy barrier of the dislocation decomposition. The mechanism that the ultrafine-grained metal material has both high strength and plasticity owing to the structure transformation of the pile-up of the GBD at the triple junction of the grain boundary is revealed.