Abstract: In this paper, the large thickness surfacing test was carried out by GTAW process, the welding hot crack of Inconel 625 alloy was studied, and the cracking mechanism of ERNiCrMo-3 welding wire surfacing metal was explained. It was indicated that the fabricated Inconel 625 sample consists of cellular dendritic which grew epitaxially from the substrate. Laves(Ni,Fe,Cr)₂(Nb,Ti,Mo) phase, MC-type carbide and acicular δ(Ni₃Nb) phases were also observed in the microstructure of as-welded sample. There are local crystal cracks in the microstructure of large thickness surfacing metal. The cracks are located between primary dendrites along the direction of columnar crystals. There is a large amount of δ(Ni₃Nb) phases, near the cracks and on the fracture surfaces. δ phases is related to the formation of cracks. There are two solidification modes for 625 alloy surfacing: (1) and (2). At the end of crystallization, mode (2) with L → γ + δ eutectic reaction has greater sensitivity to thermal cracking, resulting in cracking of Alloy 625.