The modulus of an asphalt layer subjected to repeated loadings was measured to reasonably analyze the damage development therein. The ratio of the reduction in the modulus to the modulus of the undamaged asphalt layer is used to reflect the damage of the asphalt layer; the evolution of the damage with the number of the loadings was analyzed. Full-scale accelerated pavement testing was conducted to simulate the repeated vehicle loadings on the asphalt layer. Falling-weight deflectometer, pavement seismic property analyzer (PSPA), and uniaxial compression dynamic modulus tests were conducted to obtain the modulus of the undamaged asphalt layer in the unloaded area. PSPA was used to obtain the moduli of the damaged asphalt layers in the loading area; the ratios of the reduction in these moduli to the modulus of the undamaged asphalt layer were calculated. The damage development in the asphalt layer was analyzed, and a nonlinear damage-evolution model was established, which assisted in obtaining the speed of the damage development in the asphalt layers at different depths. It is found that the S-type master curve can be used to unify the undamaged moduli derived from the three tests. The results of the field-detection method are in accordance with the predictions obtained from the laboratory master curves. In addition, the regression parameter in the damage-evolution model assists in judging the speed of the damage development in the asphalt layer. The normalization result shows that the lateral asphalt layer at the outer edge of the wheel paths and that at the center of the wheel gap experience slower damage as the depth increases. The mid-depth asphalt layers at the other positions and directions experience faster damage. In conclusion, the nonlinear damage-evolution model can provide the basis for damage development judgment and lay foundation for optimizing the maintenance timing and strategy.