Transformations of grain boundaries often strongly influence both the structure and the properties of polycrystalline and nanocrystalline materials. Thus, plastic deformation processes in fine-grained polycrystals and nanocrystalline solids are associated with transformations of grain boundaries, which crucially affect the structure and mechanical characteristics of such solids. Motion of grain boundary dislocations in plastically deformed materials is commonly considered to be the absorption of lattice dislocations by grain boundaries. In order to reveal the mechanism of motion of a low-angle symmetric tilt grain boundary (STGB) associated with the emission and absorption of lattice dislocation, the emission and evolution of a STGB under strain were simulated by phase-field crystal (PFC) model. The decay of STGB and dislocation reactions of separation, annihilation and mergence and their mechanisms were analyzed from the energy point of view, furthermore, the active energy of the dislocation separation was calculated. The research results show that the low-angle STGB is composed of pair dislocations in a line arrangement in two dimensions of triangular atomic lattice, in which there are two sets of basic Burgers vectors. The evolution process of STGB decay can be divided into six typical stages which includes the detail features as: dislocation climbs firstly along the STGB under strain, then the dislocation occurs to break up into two new dislocations after it gets enough energy to overcome the active potential barrier of dislocation, at this time the STGB emits pair dislocations to move in gliding in grain instead of climbing along STGB; gliding for while, the dislocation crosses the grain until it is annihilated by another dislocation at the STGB right in the front, i.e. the Grain boundary absorbs or merges the gliding dislocation. The remain of dislocation in the STGB can still climb along the grain boundary in which splits off again into two dislocations when it gets enough energy, at the same time it looks as if STGB emits the dislocations and changes the dislocation movement from climbing to gliding again. The dislocation continues gliding until it meets another gliding dislocation in grain to be annihilated, finally the total dislocations are annihilated and the STGB disappears. The two grain systems with STGB become one grain system. The two sets of basic Burgers vectors of lattice dislocation in triangular lattice can validly be used to express the dislocation reaction of emission, separation, mergence, absorption, annihilation, and also can reveal the creation of new Burgers vector and the annihilation of old Burgers vectors and mechanism of the directional change of Burgers vectors during the dislocation reaction.