Diurnal fluctuations in algal growth and decline are essential for understanding the dynamics and predicting algal blooms accurately in large river systems. This study analyzed environmental monitoring data during a spring algal bloom event from February to March 2022 in the Middle-lower Hanjiang River, China, and investigated the driving factors of diurnal variations. An improved coupled water quantity-quality-ecology model incorporating diurnal cycle was developed by proposing a novel light effect equation and integrating high-resolution hourly data, based on previous research and model calibration. This model can simulate river algal blooms and reveal the combined effects of the diurnal cycle on the algal growth and decline process. Model validation at the Zongguan cross-section showed satisfactory performance with the coefficient of determination (R