The dynamic mechanical properties and ballistic impact property of α+β region and β region forged TC21 titanium alloy were studied using Split Hopkinson Pressure Bar and ballistic impact experiment. The results show that in the dynamic compression tests, the α+β region forged TC21 alloy has higher dynamic strength but lower dynamic failure strain than the β region forged TC21 alloy; in the ballistic impact experiment, both the α+β region and β region forged TC21 alloy do not demonstrate improved ballistic impact property as compared to the mill-annealed TC4 alloy, which is considered to be attributed to the similar failure mechanism analyzed. In the bimodal microstructure, spall fragments induced by ductile hole formation is the failure mode; while in the lamellar microstructures, the TC21 targets fail by the type of brittle fragmentation. Further post ballistic metallurgical observations find that failure mechanism involved in TC21 alloy is facilitated by the initiation and propagation of adiabatic shear bands and cracks induced by adiabatic shear bands.