The full lamellar structure evolution and impact toughness of TC21 titanium alloy were studied by using optical microscope(OM), field emission scanning electron microscopy(FESEM) and instrumented charpy impact testing machine under different annealing temperatures. The results show that multi-level full lamellar structure can be obtained when TC21 titanium alloys are annealed at 720 ℃, 770 ℃ and 820 ℃ after solution treatment at 980 ℃. Additionally, the thickness of α platelets and grain boundary α phase, as well as the size of α colonies increase with the increase of annealing temperature, while the size of β grain remains unchanged. Moreover, the fraction of low angle boundaries increases also in the multi-level microstructure as increasing annealing temperature, resulting in the increase in crack initiation energy and crack propagation energy of TC21 titanium alloy during impact test, as well as the fraction of crack propagation energy increasing. With the increase of annealing temperature, the fracture mechanism gradually changes from transgranular to intergranular and along colony boundary.