This work investigated the effects of adding 1 at.% early transition metals (M = Ti, V, Cr, Zr, Nb, Mo) on the melt-spun structure, crystallized microstructure, and magnetic properties of Fe_84.5B₁₃Cu_1.5M₁ alloys, while exploring the mechanisms of different M elements in regulating the alloy structure and magnetic performance. Results indicate that except for the M = Zr alloy presenting fully amorphous state in as-spun condition, other alloys all contain pre-existing α-Fe grains with average grain sizes (d_α-Fe) smaller than 10 nm and high numerical density (N_d) distributed in amorphous matrices. M doping could reduce both N_d and d_α-Fe of pre-existing α-Fe phases to varying degrees, with reduction effectiveness following the sequence: Cr < V < Mo < Nb < Ti < Zr. This trend positively correlates with the enhanced glass-forming ability derived from increased atomic size mismatch and negative mixing enthalpy induced by M elements. M doping significantly influenced the α-Fe phase/amorphous-nanocrystalline composite structure and magnetic properties after heat treatment. Compared with Fe_85.5B₁₃Cu_1.5 alloy, alloys with M = V/Cr/Nb/Mo exhibited reduced average α-Fe grain sizes (D_α-Fe) and coercivity (H_c), while M = Ti/Zr counterparts showed increased D_α-Fe and H_c. All doped alloys demonstrated slightly decreased saturation magnetic induction (B_s). Notably, the Mo-doped alloy achieved optimal nanocrystalline structure and soft magnetic properties, showing D_α-Fe =14.9 nm, H_c =8.3 A/m and B_s =1.84 T, which significantly outperformed the reference alloy"s 17.9 nm, 22.1 A/m and 1.90 T. Mo doping attained optimized matching between N_d and d_α-Fe of pre-existing α-Fe grains in melt-spun alloys, which enhanced the coordinated intergranular competitive growth effects during thermal crystallization. This mechanism effectively refined the nanocrystalline structure, reduced magnetocrystalline anisotropy, and consequently improved soft magnetic properties.