Ti-5Al-5Mo-5Cr-2Zr-xNb with different Nb (abbreviated as Ti-5552-xNb, x=3, 6, 9, 12, wt.%) contents were stretched at 923 K to study their superplastic behavior and mechanical properties below recrystallization temperature. The microstructure of as-cast Ti-5552-xNb alloy is consisted of a single β phase, and the β grain size increases slightly with the increase of Nb content. The thermal effect in the process of high temperature drawing leads to the precipitation of α phase. The addition of Nb in Ti-5552 titanium alloys reduces the α/β phase transformation temperature, which causes a decrease in the volume fraction of α phase. Reducing the α phase content reduces incompatibility, but too low a proportion of α phase will lead to premature fracture, so tensile strength and plasticity firstly increase and then decrease. The results show that Ti-5552-9Nb titanium alloy shows the best tensile strength (307.2 MPa) and superplasticity (106%). The superplastic mechanism of Ti-5552-9Nb alloy is mainly caused by relative sliding of β grain boundaries and dislocation movement.