Grain size is a basic characteristic of sediments, which can reflect the dynamic conditions of sediment transport and sedimentary processes. Therefore, it has been widely used in palaeoenvironmental studies. However, due to the influence of various factors on the depositional environment in the basin, and the complexity of provenance and depositional dynamic conditions, it is difficult to understand the implications of each grain size component only based on grain size parameters. By extracting and analyzing the environmental-sensitive grain size components of sediments, it is possible to accurately reconstruct the hydrodynamic conditions and regional climate during geological period. In this study, the Miocene fluvial-lacustrine sedimentary deposits (~22.8-9.0 Ma) along the Tiejianggou section at the Subei Basin were analyzed by using a depositional grain-size end-member analysis model, and four end-members that could represent the depositional environments were obtained. Through comprehensive analyses of sediment grain size characteristics and end-member components, the sedimentary processes along the Tiejianggou section can be divided into four stages. (1) During ~22.8-21.4 Ma, the end-member component one (EM1) (representing lacustrine fine silt) and EM2 (representing lacustrine medium silt) wavelike decreased, while EM3 (representing river coarse silt) and EM4 (representing fan deltaic sandy) wavelike increased, the grain size coarsened upward and poorly sorted, indicating the depositional environment were dominated by low-energy fan deltaic facies with great fluctuations. (2) During ~21.4-17 Ma, EM1 and EM2 significantly increased, while EM3 and EM4 decreased, the grain size fined upward and was dominated by static deposition of fine sand with good sorting, indicating the strengthening influence of lakes and the depositional environment transited to a stable shallow lake or lacustrine face. (3) During ~17-14 Ma, EM1 decreased, EM2 and EM3 were relatively stable with EM3 accounting for the highest proportion, EM4 increased slightly, the grain size coarsened again and were dominated by coarse silt from river suspension input with poor sorting, indicating an unstable depositional environment with low-energy fluvial deposition. (4) During ~14-9 Ma, EM1 wavelike decreased, EM2 and EM3 were relatively stable, and EM4 continued increasing. The grain size significantly coarsened and was dominated by coarse sand from saltation and suspension input and the sorting was poor with enhanced hydrodynamic forces, indicating the most unstable depositional environment that was dominated by high-energy fan delta phases. The depositional environment of the Tiejianggou section transitioned from the early fan delta to the stable lacustrine and finally evolved into the high-energy fan delta phase, and the staged evolution of hydrodynamic conditions from weak to stable, and to enhanced ones. By comprehensive analyses of previous results, we concluded that the depositional environment and hydrodynamic changes during the middle to early Miocene (~22.8-14 Ma) in the Subei Basin were mainly dominated by the regional warm and humid climate, while the high-energy environment during the middle to late Miocene (since ~14 Ma) was jointly dominated by the uplift of the Tibetan Plateau and regional aridification.