Traffic speed deflectometer (TSD) has been widely used in pavement maintenance, yet an indicator system for identifying layer-specific structural strength remains lacking. On the basis of a three-dimensional finite element model incorporating viscoelastic damage, this study analyzed how surface damage density and the modulus of the base and subbase affect deflection slope. Two layer-sensitive indicators of structural strength were proposed: S₁₀ (deflection slope 10 cm ahead of the load center) for characterizing the surface layer, and S₃₀ (deflection slope 30 cm ahead of the load center) for reflecting the structural strength of the base layer. Simulation results show that S₁₀ is highly sensitive to surface damage, with maximum variation amplitude exceeding 600%. S₃₀ correlates strongly with base modulus changes, exhibiting excellent recognition capability. Measured TSD data from a typical expressway were used to validate the indicators. The results indicate that S₁₀ and S₃₀ exhibit distinct layer response characteristics, enabling effective differentiation of different lane conditions. The research provides a new path for mechanical interpretation of raw TSD data and establishes an applicable parameter system and a theoretical basis for evaluating the layer-specific structural strength of asphalt pavement.