Investigations of microstructural evolution in undercooled metallic melts have been extensively conducted in different alloy systems, such as eutectic, peritectic and metallic compound, however, few reports about undercooled superalloys were found up to date.  In this paper, DZ125 superalloy melt was undercooled by the method of molten salt purification combined with cycled superheating under Ar atmosphere, and a substantial undercooling up to 180 K was obtained. Based on the microstructural observation as well as the calculations of the dendritic tip radius, the dendrite growth velocity and undercooling with BCT (Boettinger, Coriell, Trivedi) dendrite growth model, the solidification behavior and structure transition mechanisms were systematically investigated. The results indicate that the microstructure undergoes three sequent transitions in the achieved range of undercooling range ΔT. When ΔT<48 K, the dendrite growth is mainly controlled by solute diffusion, and the solidification structures are the common dendrites. When 48 K≤ΔT< 85 K, the dendrites is remelted and the first kind of granular grains forms because of the serious remelting produced by recalescence. When 85≤ΔT<160 K, the effect of solute diffusion is weakened by solute trapping that results from high dendrite growth velocity, thermal diffusion plays the dominate role at the same time, the remelting effect decreases to a low level, and ripenings of dendrite are restrained, the granular grains turn into highly developed fine dendrites. WhenΔT>160 K, the second kind of granular grains is formed, the transition is caused by the stress originating from the rapid solidification contraction, which results in the recrystallization of the fine dendrite.