The dynamic softening behavior of coarse-grained Ti40 alloy during superplastic deformation was studied by means of tensile tests as well as TEM observation and EBSD analysis. The results show that coarse-grained Ti40 alloy exhibits good superplasticity in most test condition and the maximum elongation 436% was obtained at the condition of 840oC, 1×10-3s-1. The deformation conditions can be divided into three areas: none superplasticity area, dynamic recovery area and dynamic recrystallization area based on the Zener-Hollomon factor and elongation combined with the microstructure analysis. The critical strain model and dislocation density evolution model of dynamic recrystallization were established based on Sellars model and KM equation respectively. The dynamic recovery mechanism is dominated by dislocation motion—dislocation cells—polygonization—subgrain formation, while the recrystallization mechanism is mainly the continous dynamic recrystallization resulting from the subgrains rotation leading to the formation of high-angle grain boundaries.