The evolution of the hydrological regime in the middle and lower reaches of the Han River, under the combined influence of the Danjiangkou Reservoir and the South-to-North Water Diversion Project's Middle Route (SNWDP-MR), is crucial for the management of reservoir operations, flood and drought prevention, and water resource management. This study selects historical hydrological data from four typical hydrological stations along the mainstream of the middle and lower reaches of the Han River. By utilizing methods such as statistical analysis of characteristic factors and IHA-RVA evaluation of hydrological alteration, the study examines the evolution of the hydrological regime under the comprehensive impact of these projects. The results indicate that after the operation of the SNWDP-MR, the water levels at various stations along the river showed a general declining trend, with the intra-annual distribution of water levels becoming more uniform. Influenced by the construction of the Danjiangkou Reservoir and water transfer from the Middle Route, the annual average discharge and extreme value ratios at each station decreased in stages, with the largest reduction observed at the Huangjiagang station, where the decrease reached 33.5%. The intra-annual distribution of discharge also exhibited a trend toward uniformity. After the construction of the Danjiangkou Reservoir, the overall hydrological alteration degree at the Huangjiagang station reached 55%, increasing to 67% after the SNWDP-MR became operational. Similarly, the hydrological alteration degrees at the Huangzhuang station during the same periods were 53% and 58%, respectively, with both stations exhibiting moderate overall hydrological alteration. The methods and conclusions of this study can provide scientific support for flood and drought disaster prevention, water resource management, water ecological protection, and the scientific scheduling of inter-basin water diversion projects within the basin.