A large undercooling level up to 550K(0.386TE) was achieved in eutectic Ni78.6Si21.4 melt by the combination of molten-glass and cyclic superheating. A microcrystalline structure is obtained at large undercooling. Surprisingly, the morphology of α(Ni) phase transits from the non-faceted phase to faceted phase at large undercooling of 390K. Based on the classical nucleation theory and transient nucleation theory, the process of microstructure evolution and competitive nucleation was analyzed, and the refinement of crystal structure is determined by the high nucleation rate under large undercooling.