Abstract: The evolution of a special "black crystal" structure in a cast and rolled Fe-Ni based superalloy was studied, and the J-matpro software and DICTRA software were used to simulate the phase transformation of the alloy and the element diffusion process during heat treatment, respectively. Finally, the heat treatment process was carried out to eliminate the "black crystal" structure. The results show that the special "black crystal" structure in the alloy have two reasons, one is the serious segregation of Nb and Cr elements at the end of solidification, and the other is the difference in corrosion resistance, which leads to obvious difference in the metallurgical color of the grains after corrosion. The "black crystal" structure is different obviously with the MC carbide and brittle Laves phase in the shape and size. After rolling, the grains are refined, the(111)surface polar density increases, the volume fraction of "black crystals" decreases from 24.69% of the as-cast state to 12.09%,and the self-corrosion current decreases from 0.556 μA in as-cast state to 0.055 μA, the corrosion resistance is improved. The alloy phase transition thermodynamics and elemental diffusion dynamics analysis show that serious segregation of Nb, Cr elements in the solidification process occurs, Nb element segregation can be eliminated by 1150 ℃ homogenizing annealing for 15 min. The test shows that when the rolled alloy is heat treated at 1150 ℃/15 min, Nb and Cr elements are evenly distributed, the self-corrosion current is further reduced to 0.048 μA, the corrosion resistance is improved, and the black crystal is eliminated.