Abstract: To investigate the characteristics of microstructure and properties of welded joints of TC21 high-strength titanium alloy by rotary friction welding and to further optimize the mechanical properties of the joints, welded joints under different conditions were prepared, and the mechanical property characteristics and the microstructure evolution rules and characteristics of the joints in the as-welded state and post-weld solution aging state were studied. The average tensile strengths of the as-welded joints and post-weld solution aging-treated joints were 1 124 MPa and 1 163 MPa, respectively. The impact absorbed energy of the weld zone of the as-welded joint was 12.2 J, and that of the post-weld solution aging-treated joint was 29.2 J; after the solution aging treatment, the impact absorbed energy of the weld zone increased by 139%. The results indicate that after welding, a large amount of acicular martensitic α′ phase precipitates in the weld zone, which leads to an increase in the microhardness of the weld zone; however, after the solution aging treatment, the microstructure of the weld zone transforms into rod-like secondary α phase, grain boundary α phase, and intragranular secondary α phase with a fine structure, and the hardness of the weld zone decreases compared with the as-welded state. The change in the microstructure of the weld zone after the solution aging treatment improves the hardness uniformity of the joint, and the change in the morphology and size of the α phase/α′ phase significantly improves the impact toughness.