Eutectic multi-principal element alloys combine the good workability of eutectic alloys with the excellent comprehensive properties caused by the high-entropy effects of multi-principal element alloys. By coupling various microstructure regulation methods, more diverse microstructural characteristics are expected to be exhibited, and its performance potential can be further explored. On the basis of this, static magnetic field and deep undercooling technology were combined to systematically investigate the non-equilibrium solidification behavior and microstructural evolution law of the FeCoNiB0.65 alloy. The results demonstrate that with increasing undercooling, the primary structure undergoes a morphological transition from α-M dendrites to seaweed-like eutectic dendrites, and the two phases inside the seaweed transition from α-M/M23B6 to α-M/M3B. The alloy matrix frequently experiences competitive formation between the M23B6 phase and the M3Bphase, which is related to the critical nucleation barrier that needs to be overcome during solidification. The coexistence of two phases in the matrix at ΔT=60 K is related to the solid-state phase transition from incomplete decomposition of the M23B6 phase to the M3Bphase. Both the M23B6 and α-M phases have FCC crystal structures and exhibit significant coupled growth characteristics, forming a cube-cube orientation relationship during solidification.