The method of molten salt denucleating was applied to study the rapid solidification of IN718 superalloy. The highest undercooling up to 250 K was produced, and the microstructure evolution with undercooling was investigated. Based on the calculations of the dendritic tip radius, the dendrite growth velocity and undercooling, the solidification behavior and structure transition mechanisms were analyzed by adopting BCT dendrite growth model. Result show that the solidification morphologies of the undercooled IN718 superalloy are dependent on the degree of undercooling. Within the achieved undercooling, 0~250 K, the solidification microstructure of IN718 undergoes twice grain refinement. When 0 K≤ΔT＜63 K, it is the first grain refinement; when 63 K≤ΔT＜90 K, with the increasing of undercooling, grain coarsening comes into being; when 90 K≤ΔT＜250 K, it is the second grain refinement, and aforementioned dendritic structure transforms into granular grain with average grain size about 5.5 μm