Abstract: The toughening mechanism of the weld metal by self shielded flux cored wire was introduced, the influence of alloy elements, inclusion and rare earth element on weld-seam toughness were analyzed. And the results indicated that coarse M-A structures in weld-seam caused the discreteness of weld toughness. The wire with proper chemical composition and appropriate thermal input were used, M-A structures can be reduced and refined and the toughness will become stable.