The rapid advancement of modern industry and information technology has led to higher demands for efficiency, power density and reduced energy consumption in power electronic devices, with soft magnetic materials playing a critical role in their performance. Traditional Fe–Si–B–Nb–Cu nanocrystalline alloys (Finemet) have been widely employed due to their excellent overall soft magnetic properties, including low coercivity (H_c), high permeability and low core loss. However, their relatively low saturation magnetic flux density (B_s) remains a limitation. Developing new nanocrystalline alloys with enhanced B_s and industrial applicability is thus of great importance. In this study, with the aim of producing high-B_s nanocrystalline alloys possessing good manufacturability, minor Ga was added to a Fe_81.5Si₄B₁₃Cu_1.5 alloy. The effects of Ga addition on the as-quenched and annealed structures, as well as the soft magnetic properties, were investigated. The results show that the incorporation of Ga changes the as-spun structure of the Fe_81.5−xSi₄B₁₃Cu_1.5Ga_x (x = 0–2.0) alloys from a fully amorphous state to one containing a high number density (N_d) of pre-existing α-Fe grains, with average grain sizes (d̅_α-Fe) below 7 nm dispersed within the amorphous matrix. Both N_d and d̅_α-Fe increase progressively with increasing Ga content. Ab initio molecular dynamics simulations reveal that Ga addition lowers the atomic packing density and weakens interatomic chemical bonding within the alloy, thereby reducing long-range atomic diffusion resistance during rapid solidification and consequently decreasing the glass-forming ability (GFA). The reduced GFA, along with the formation of (Cu, Ga) clusters due to Ga-promoted Cu aggregation, facilitates the precipitation of pre-existing α-Fe grains. Upon annealing at a low heating rate, the Ga-containing alloys develop a fine and uniform dual-phase structure comprising amorphous matrix and α-Fe nanocrystals, resulting in excellent soft magnetic performance. Specifically, the alloy with x = 0.5 exhibits an average α-Fe grain size of 16.3 nm, B_s of 1.77 T, H_c of 9.0 A/m, and an effective permeability of 8,700 at 100 kHz, significantly outperforming the x = 0 alloy, which has corresponding values of 47.6 nm, 1.78 T, 213.6 A/m and 500, respectively. The competitive growth between pre-existing and newly formed α-Fe grains during annealing effectively suppresses grain coarsening, leading to a refined nanocrystalline structure in the Ga-containing alloys. This refined structure reduces the average magnetocrystalline anisotropy and promotes the formation of regular magnetic domain structures, thereby significantly improving magnetic softness.