Abstract: Influenced by the local angle of attack of the air rudder, a complex separation and reattachment flow will form at the position of the full-motion rudder shaft thermal-protection ring during a hypersonic flight, accompanied by severe aerodynamic heating loads, which is the weak link in the thermal protection design of the air rudder. Taking the flat plate/air rudder shaft thermal-protection ring as the research object, the numerical calculation method was used to study the influence rule of different rudder deflection angles and geometric parameters of the thermal-protection ring's annular gap on the flow and aerodynamic heating. The numerical calculation was based on the unstructured hybird gird finite volume method. The calculation results and analysis show that the rudder deflection has the greatest effect on the heat flux distribution in the thermal-protection ring. Under the condition of rudder deflection, a reattached flow and high flux region will form at the chamfer of the thermal-protection ring. The area of the high flux region and the flux peak are proportional to the rudder deflection angle. When the rudder deflection angle equals 0°, the airflow at the Z=0 mm cross-section of the annular gap will flow upward into the gap at the bottom of the rudder surface. When the rudder deflection angle is greater than 0°, the incoming flow in the gap at the bottom of the rudder surface will form a vortex in front of the chamfer of the thermal-protection ring, and at the same time the airflow on the flat plate enters the annular gap downward and forms a vortex in the direction of the gap depth. When the rudder deflection angle and the gap width are fixed, the gap depth mainly affects the heat flux at the chamfer of the thermal-protection ring and the annular gap windward side. As the gap depth increases, the flow structure at the Z=0 mm cross-section of the annular gap develps from 2 vortices to 3 vortices. When the rudder deflection angle and the gap depth are fixed, the gap width mainly affects the heat flux at the annular gap windward side. As the gap width increases, the vortex at the Z=0 mm cross-section of the annular gap develops more fully, and the flow structure in the depth direction develops from 3 vortices to 2 main vortices and 2 small vortices at the bottom of the annual gap.