The Traffic Speed Deflectometer (TSD) is increasingly utilised as a nondestructive tool for measuring continuous deflections in asphalt pavements. These deflections are calculated from real-time measurements of deformation velocities recorded using the device’s laser vibrometer, combined with the vehicle's travelling speed. However, existing methods for calculating TSD deflections are limited by accuracy and computational efficiency constraints. To address these issues, an improved deflection calculation method was developed. First, finite element (FE) simulations were performed to clarify the deflection slope distribution characteristics of typical flexible and semi-rigid pavements under various conditions. Various fitting curves were then applied to the deflection slope data to identify the most suitable models, and an improved curve area integration method was employed to calculate the corresponding deflection values. Additionally, the impact of different subgrade moduli on the far-end deflection basin of semi-rigid pavements was analyzed, allowing for the determination of the zero-response position of the deflection slope, leding to a proposed correction method for TSD measurements. Finally, the improved deflection calculation method was validated through comparative error analysis with TSD-measured values and FE model results, demonstrating its accuracy and reliability. The findings are expected to support more precise TSD deflection basin determination, improving pavement condition assessment.