The invention relates to a method and device for refining crystal particles in additive manufacturing. According to the method, in the fusion additive manufacturing process, a semisolid formed blank stirring method is adopted, dendrites/branch crystals are smashed under stirring, the nucleation rate is increased, the crystallization rate is increased, the crystal particles are refined, two-side-face and vertical-direction shaft shoulder shape follow-up restraint modes are synchronously combined to restrain a blank, the problem that in the stirring process, the blank flows and deviates from thedigital-analog or forming is discontinuous due to stirring is prevented, and side-face shape follow-up restraint and stirring movement are both synchronously conducted along with additive manufacturing movement. Stable-property, efficient, high-precision and low-cost additive manufacturing of complex parts made of materials difficult to form can be achieved.