Abstract: The stiffness of the base layer significantly influences the mechanical state and failure modes of asphalt pavement structures. To investigate the mechanical behavior of asphalt pavements on rigid bases and improve the structural performance of rigid-flexible composite pavements, this study derived a viscoelastic-plastic constitutive model for asphalt mixtures, established a thermo-mechanical coupling model for composite pavements, extracted the temperature-modulus fields within the asphalt layer, and analyzed the mechanical response under coupled thermal-mechanical loading. A modulus gradient structure for composite pavements was proposed. The results indicate that significant temperature gradients exist within the asphalt layer under environmental thermal conditions, leading to spatiotemporal modulus gradients. The compressive-shear mechanical behavior of the asphalt layer in composite pavements under thermo-mechanical coupling was clarified, emphasizing the critical role of shear stress in asphalt layers over rigid bases during design. When the base layer modulus approaches the surface layer modulus and the asphalt layer modulus increases gradiently with depth, the shear stress in the asphalt layer decreases. Therefore, introducing a modulus transition layer between the asphalt surface and rigid base is essential to mitigate modulus mismatch and reduce shear stress. Based on the response surface model (RSM) with the minimization of maximum shear stress as the optimization target, the optimal modulus transition structure was determined: a 4 cm upper asphalt layer, an 8 cm transition layer with a modulus twice that of the asphalt layer. This configuration reduced the shear stress in the upper layer and transition layer by ??14.3%?? and ??20.5%??, respectively, compared to a structure without a transition layer. The findings provide theoretical support for the structural design and material development of rigid-base asphalt pavements.