<p>The effects of partial substitution of Si with P on the as-spun and crystallized structures, as well as on the static and high-frequency magnetic properties of low-Nb-content Fe₇₈Si_{12.5-<i>x</i>}B₇Cu₁Nb_1.5P_<i>x</i> (<i>x</i> = 0-4) alloys were investigated. The correlations among P content, crystallization kinetics, nanocrystalline and magnetic domain structures, and magnetic properties were elucidated. The incorporation of P enhances the amorphous-forming ability, suppresses the formation of α-Fe texture at the surface of as-spun ribbons, and yields a fully amorphous structure. After annealing, all ribbons exhibit a dual-phase structure consisting of α-Fe nanograins embedded in an amorphous matrix. The nanocrystalline structure of the P-containing alloys is markedly refined, accompanied by a pronounced improvement in magnetic softness. Notably, the nanocrystalline alloy with <i>x</i> = 2 exhibits an average α-Fe grain size, coercivity, saturation magnetic flux density, effective permeability at 100&#xa0;kHz, and core loss at 0.2&#xa0;T/100&#xa0;kHz of 17.2&#xa0;nm, 2.7 A/m, 1.51&#xa0;T, 18000, and 353&#xa0;kW/m³, respectively, significantly superior to those of the P-free alloy (32.4&#xa0;nm, 45.9 A/m, 1.53&#xa0;T, 2700, and 722&#xa0;kW/m³, respectively). The substitution of an appropriate amount of P for Si promotes α-Fe nucleation during annealing, which induces strong competitive growth among α-Fe nanograins and suppresses grain coarsening, thereby leading to a refined nanocrystalline structure. Accordingly, more regular magnetic domains are formed, which results in substantially enhanced magnetic softness. The reduced magnetic hysteresis and increased electrical resistivity contribute to decreased core loss. These findings demonstrate the critical role of P in optimizing magnetic softness and provide a promising pathway for the development of cost-effective nanocrystalline alloys with high saturation magnetic flux density.</p>