Abstract: Inflatable decelerator is subject to significant aerodynamic and aerothermal loads during hypersonic deceleration. To optimize the mechanical characteristics of inflatable decelerator under complex loads, four inflatable decelerator models with different sized outer diameter inflatable tours were established based on NASA’s project IRVE-3. A unidirectional fluid-solid-thermal coupling analysis under Ma=5 flight condition was carried out. The effects of the outer diameter size of inflatable tours on the mechanical characteristics and temperature distribution of the inflatable decelerator under aerodynamic, aerothermal, and superimposed loads were compared. The results show that the smaller the outer diameter of the inflatable tour is, the smaller the overall deformation, the higher the temperature, and the higher the stress in the thermal protection layer. The larger the outer diameter is, the greater the stress concentration of the inflatable tour. It indicates that either too large or too small size of the outer diameter of inflatable tours has a negative impact on the mechanical characteristics of the inflatable decelerator. The proposed study may provide a reference for the inflatable decelerator design in the future.