The superplastic deformation behavior and mechanism of burn-resistant Ti40 alloy with large grains at elevated temperature were investigated by OM and TEM. Results show that Ti40 alloy exhibits excellent superplastic behavior in the strain rate range from 5×10-5 to 1×10-2 s-1 at 920 ℃. All of the tensile elongations exceed 250% while the strain rate sensitivity index m is over 0.3. Optical microstructure shows that the large equiaxed gains are refined after superplastic deformation. TEM observation indicates that an unstable subgrain boundary formed by dislocation during superplastic deformation. The subgrain boundaries were transformed into low- and even high-angle grain boundaries by absorbing gliding dislocations. The superplastic behavior of large-grained Ti40 alloy could be explained by dynamic recovery and recrystallization.