Friction stir lap welding of AA2195 Al-Li alloy and Ti alloy was conducted to investigate the formation, microstructure, and mechanical properties of the joints. Results show that under different welding parameters, with the decrease in welding heat input, the weld surface is smoother. The Ti/Al joint interface is flat without obvious Ti and Al mixed structure, and the hook structure is not formed under optimal parameters. Due to the enhanced breaking effect of the stirring head, the hook structural defects and intermetallic compounds are more likely to form at the Ti/Al interface at high rotational speed of 1000 r/min, thereby deteriorating the mechanical properties of joints. Decreasing the heat input is beneficial to hardness enhancement of the aluminum alloy in the weld nugget zone. Under the optimal parameters of rotation speed of 800 r/min and welding speed of 120 mm/min, the maximum tensile shear strength of joint is 289 N/mm.