In this work, Grain boundary engineering (GBE) is used to optimize and control the microstructure of this alloy approach to advancing without changing chemical composition of GH3625 alloy, thereby, improving the high temperature microstructure stability and service performance reliability of the alloy. The effect of thermal-mechanical processing on the grain boundary character distribution (GBCD) of GH3625 superalloy was investigated by means of the electron backscattered diffraction (EBSD) technique and orientation image microcopy (OIM). The results show that the optimization of the grain boundary character distribution (GBCD) of GH3625 superalloy is mainly achieved byΣ3_n grain boundaries formed during the recrystallization process, and is mainly affected by the cold deformation and annealing process. The length fraction of low ΣCSL grain boundaries in GH3625 superalloy decreases with the increase of cold deformation and increases with the annealing temperature. Meanwhile, the length fraction of low ΣCSL (coincident site lattice, Σ ≤29 by Palunbo-Aust criterion) grain boundaries increase to more than 63.16 % by thermal-mechanical processing after 35 % cold deformed and subsequent annealing at 1120 ℃ for 15 min. In addition, large sized grain-clusters appear in GH3625 superalloy, and boundaries have Σ3_nmisorientations inside the grains-cluster. The size of the grain-clusters and the amount of Σ3_n grain bounaries inside the grains-cluster decreases with the increase of cold deformation and increases with the annealing temperature.