The precipitation behavior of σ phase and its effect on mechanical properties of ER347 deposited metal during long-term aging at 750 ℃ are studied. The results show that the ferrite content,hardness and impact toughness of ER347 deposited metal are closely related to the aging time. When ER347 deposited metal is aged at 750 ℃,with the extension of aging time, σ phase precipitation increases,hardness increases,while the ferrite content and toughness decrease. ER347 deposited metal has good as-welded impact toughness,but the toughness decreases after aging treatment. The root cause of the embrittlement is that δ-ferrite decomposes to form a hard and brittle σ phase,which significantly reduces the toughness of ER347 deposited metal. There are a large number of high-energy σ/γ2 and δ/σ/γ2 incoherent interface inside（σ +γ2） structure,and the σphase itself is hard and brittle, which is easy to act as a potential crack source,under high strain rate,brittle cracking occurs and spreads rapidly, resulting in the deposited metal toughness decreased.