Abstract: Currently, research on the quantitative relationship between the transmission efficiency of light guide tubes and their aspect ratio is insufficient, leading to a lack of clear basis for selecting the optimal aspect ratio during the architectural design phase. This paper focuses on the functional relationship between the transmission efficiency of light guide tubes and their aspect ratio. Based on the latitude characteristics and light climate conditions of the Guangzhou region, under standard overcast conditions, the transmission efficiency of light guide tubes under 16 different aspect ratio conditions was simulated using HOLIGILM software, and a numerical relationship model was established through regression analysis. The results indicate that a higher internal reflectance of the light pipe reduces energy loss during multiple reflections, thereby effectively mitigating the decline in transmission efficiency caused by increasing aspect ratio (L/D). A strong exponential negative correlation is observed between transmission efficiency and aspect ratio, with coefficients of determination (R²) exceeding 0.99, demonstrating excellent model interpretability. These findings provide theoretical support for parameter optimization during the design phase and contribute to improving the performance of light pipes in building daylighting systems. The results offer valuable guidance for light pipe design in South China and other regions at similar latitudes.