Microstructure evolution principles of Ti-6Al-4V laser welding joints during superplastic deformation were analyzed, and some involved parameters were proposed to evaluate the forming process. The results indicate that during superplastic deformation the α′ martensite transformed into α+β phases, and needle-like structure transformed into lamellar structure. During superplastic deformation equiaxial rate increased with straining, and also increased with increasing forming temperature and decreasing strain rate. Average grain size was introduced to characterize the microstructure transition extent of HAZ, average grain size increasing with straining, and also increasing with increasing forming temperature and decreasing strain rate.