Abstract: The stiffness of base layers significantly influences the mechanical response and failure mode of asphalt pavement structures. To investigate the mechanical behavior of the asphalt layer and enhance its structural performance, this study systematically analyzed the compression-shear behavior within the asphalt layer and developed a novel modulus gradient design. A viscoelastic-plastic constitutive model was derived for asphalt mixtures. Subsequently, a thermomechanically coupled model specific to composite pavements was established. This model facilitated the extraction of the temperature field within the asphalt layer under environmental conditions, enabling the direct characterization of its modulus gradient field owing to the high-temperature dependence of asphalt. Comprehensive coupled thermomechanical analyses were performed to elucidate the mechanical behavior of composite pavements under combined thermal and loading stresses. Results revealed a distinct temperature gradient within the asphalt layer in response to ambient conditions. This temperature gradient inherently induced time- and space-dependent modulus gradients in the asphalt material. The analysis clearly identified the critical compression-shear mechanical behavior exhibited by the asphalt layer in the composite pavement under thermomechanical coupling, indicating that the shear stress within the asphalt layer atop the rigid base warrants paramount consideration during structural design. This study demonstrates that minimizing the shear stress within the asphalt layer can be achieved under two key conditions: (1) when the base modulus closely approaches the modulus of the asphalt layer and (2) when the asphalt layer modulus gradually increases with depth. These findings strongly support the necessity of incorporating a modulus transition layer between the asphalt layer and rigid base. This transition layer effectively mediates the modulus disparity between the softer asphalt and rigid base, thereby reducing the shear stress concentrations within the asphalt. To optimize this modulus transition design, a response surface methodology (RSM) was employed, with the optimization objective set to minimize the maximum shear stress occurring within the asphalt layers (surface and transition). RSM optimization yielded the following optimal modulus transition structure parameters: a 4 cm thick upper asphalt surface layer, an 8-cm thick transition layer, and a transition layer modulus requirement of twice the modulus of the asphalt surface layer. Implementing this optimized design significantly reduced the shear stress levels. Specifically, the maximum shear stress in the upper surface layer was reduced by 14.3%, and the maximum shear stress in the transition layer was reduced by 20.5% compared to equivalent locations in the composite pavement without a modulus transition layer. This study provides an in-depth understanding of the thermomechanical behavior, particularly the compression-shear response, of composite pavement asphalt layers and successfully introduces a targeted modulus transition layer design strategy. The optimized modulus-gradient structure offers a practical solution for mitigating critical shear stresses, ultimately contributing to enhanced durability. These findings provide fundamental insights for understanding the structural mechanics and guiding the development of asphalt mixtures for rigid base pavements.