The poor oxidation resistance of titanium and its alloys limits their use at elevated temperature. To solve this problem, a large amount of surface engineering techniques to produce anti-oxidation coatings on titanium alloys were utilized. In the present research, a solid-state processing method of friction stir lap welding (FSLW) was used to fabricate Al cladding or coating on the surface of TC4 titanium alloy, with lower cost and simpler operation which are still desirable for the coating preparation on titanium alloys. The lap joint structure was smartly transformed into an interface structure of coating. In this work, the Al coating with a thickness of 500μm was fabricated via multi-pass FSLW process using a slight plunge depth of tool-pin. The mechanical milling was used as a post-treatment for a suitable coating thickness. The oxidation testing was conducted at 700℃ under air atmosphere. The microstructure, chemical composition analysis and phase determinations were performed using SEM, EDS and XRD methods. The evolutions of interlayer under the high-temperature oxidation procedure were detailed. It was found that the Ti- rich interlayer, with a thickness of 60μm, had a typical structure of mixed layers. The sufficient Al coating thickness played an important role in preventing the inter diffusion of oxygen, while the oxidation and melting phenomenon of Al coating occurred. The abundant Al content in the Al coating upper the interlayer, with a significant thickness, also benefited to the anti-oxidation performance and forming of the beneath Ti/Al interlayer at a rare oxygen environment due to the obstacle effect of the Al layer to oxygen diffusion, which exerted a main role in oxidation prevention for titanium alloy. As a result, the phases of outer surface were mainly Al₂O₃, Al₂Ti and Al₃Ti. The gradient distribution characteristic of Ti/Al interface structure occurred after the oxidation testing.