Abstract: Elastic-plastic behaviors of strength mismatched welded joints with cracks were investigated with finite element method.Effects of strength mismatching,crack length and stress-strain curve form of base metal on crack tip opening displacement were analyzed.The results show that the crack driving force increases with the increasing of strength mismatch factor,the influence of strength mismatching for welded joints with shallow cracks is more significant than those with deep cracks.When the base metal doesn't show yield platform,the cracking driving force increases monotonously with the applied strain.For the base metal with yield platform,the cracking driving force and applied strain relationship presents a stage change for over-matched or welded joint with shallow crack,which caused by that the strain hardening of base metal lags behind the deformation of weld metal.