Abstract: Microstructures of welded joints before and after superplastic deformation were observed by optical microscope, and the forming mechanism was discussed. The results indicated that increasing temperature and decreasing velocity were good for superplastic deformation, and the grains of base metal grew coarser and the phase ratio increased. Needle-like martensite phase of weld bead became shorter and wider, exhibiting globing trend under higher temperature. The maximal microhardness of cross section of weld bead after deformation is 380 HV, about 50 HV lower compares with original weld bead, which meets the requirnement of the actucal lead.