To address the impacts of sudden releases of low-temperature bottom water and sediment on downstream water temperature and aquatic ecosystems during flood season sediment flushing of the Xiaolangdi Reservoir, a water temperature model for the reservoir was developed based on the vertical two-dimensional CE-QUAL-W2 model, and the critical parameters of the model were calibrated using prototype water temperature observation data from the reservoir area. Combined with the operational regulations, water level fluctuations, and hydrological boundary conditions from 2009 to 2023, the spatiotemporal evolution characteristics of reservoir water temperature over the past 15 years were simulated. The results show that the established model demonstrates strong simulation capabilities for the water temperature dynamics and water level fluctuations in the long and narrow reservoir, and can accurately reflect the thermohydrodynamic process during actual operational scenarios. The reservoir exerts a flattening and time-lag effect on the temperature of discharged water, while the water temperature within the reservoir area exhibits a distinct seasonal stratification pattern. In winter(December to Februar of next year), the water body is well mixed, and the thermocline disappears. In spring (March to May), with increasing solar radiation, the thermocline gradually develops and expands. In summer (June to August), the high flow turbulence caused by water sediment regulation disrupts the stratification, leading to the trend toward uniform mixing of the water body. In autumn (September to November), the thickness of thermocline rapidly decreases, and stratification completely dissipates by November. The vertically averaged water temperature near the dam in the upstream area reaches its peak in August and is the lowest in January to February. The maximum difference between surface and bottom water temperatures occurs in May，while the minimum difference is observed in November. During the water sediment regulation period, the water temperature exhibits phased variations: the high flow rate weakens stratification in the early stage, the water body becomes completely mixed and isothermal in the mid stage, and the surface temperature is significantly influenced by inflow in the later stage. The water temperature changes are jointly controlled by multiple factors, including meteorological conditions, hydrological processes, reservoir operation, and physical properties of the water body.