The XRD, SEM, EBSD were used to analyze the microstructure characteristics in different zone of welded joints in the welding state and post-welding heat treatment state, and the fracture toughness and fatigue crack propagation properties of welded joints were researched. The results show that the weld zone is main consisted of recrystallization organization. The equiaxed prior α_Pphase transform the rodlike structure in thermo-mechanically affected zone. The microstructure of the heat affected zone is basically the same base metal. The acicular martensite α" phases with cross angle of approximately 60° are formed within part original β grains in thermo-mechanical affected zone and thermal affected zone. After post-weld heat treatment, the residual metastable β phase was decomposed and a large number of discontinuous microstructure were formed between lamellar α_S phase. A large number of parallel or cross distributed flake α phases and complex phase interface structure within the α grains in weld zone can effectively hinder the crack propagation and change the crack propagation path, and fracture toughness and fatigue crack propagation resistance of welded joints are improved.