The liquid-phase diffusion coefficient of L-serine is a critical parameter for studying the production processes and biological metabolism of related pharmaceuticals. To accurately determine this coefficient, our research group independently designed and fabricated a composite liquid-core cylindrical lens (SLCL-Doublet). This lens consistently demonstrated excellent refractive index resolution exceeding 0.00004 RIU while maintaining good imaging quality across various liquid samples. By leveraging the one-dimensional refractive index resolution capability of the SLCL-Doublet and applying Fick's second law, we utilized the equal-refractive-index thin-layer method to measure the liquid-phase diffusion coefficient of L-serine aqueous solutions at varying concentrations at 298.15 K. The measurement uncertainty was controlled within 0.15 × 10^-6 cm²·s^-1. This method offers advantages such as simple experimental operation, minimal time requirements, stable instrumentation, and high measurement accuracy, providing an effective solution for the rapid and accurate determination of liquid-phase diffusion coefficients.