Abstract: The water level fluctuation zone of reservoirs represents a transitional area between terrestrial and aquatic ecosystem. Clearly defining the spatial distribution characteristics of this fluctuation zone is crucial for both the protection of ecological environments and ensuring the safe operation of reservoirs. In this study, 35 large reservoirs in Sichuan Province were selected as the research objects. The gradient distribution of the landforms and the corresponding driving factors were explored within the 5-km buffer zone. Additionally, the change in explanatory power (q) was explored by progressively expanding the buffer zone range. The results indicated that: (1) The water level fluctuation range in low-slope area was primarily concentrated in the regions with a low altitude, whereas in the high-altitude and high-slope areas, the gradient distribution of water level fluctuation zone tended to be more uniform. (2) Within the 5-km buffer zone surrounding the water level fluctuation zone, the optimal parameter geographic detector was employed. It was revealed that the primary driving factors were geomorphology, annual evaporation and annual average temperature. (3) The explanatory power (q) of most factors exhibited a decreasing trend as the buffer zone was progressively expanded by 5 km increments, which indicated that the influence of the buffer zone on the fluctuation zone was distance-dependent. This study provided valuable data support and a theoretical foundation for the formulation of policies on ecological restoration and operational management of regional reservoir water level fluctuating zones.