The dissolved oxygen (DO) dynamics in narrow-deep channel-type reservoirs under frequent hydrodynamic disturbances are distinct from those in conventional deep reservoirs. Through integrated monitoring of a representative reservoir in the Min River Basin, this study reveals that DO depletion results from the synergistic interaction of three mechanisms: thermal stratification, algal-nutrient feedback, and coupled topographic-circulation. It was found that a short-term yet thick thermocline (~20 m thickness, maximum ΔT 6.1℃) serves as the primary physical barrier to vertical oxygen exchange, causing persistent bottom hypoxia (DO < 0.5 mg/L). High-frequency flood pulses (flushing rate α = 20.38) trigger abrupt mixing events, reducing vertical DO gradients by up to 79%. Algal photosynthesis leads to surface DO supersaturation (peak 9.1 mg/L), while algal sedimentation and decomposition, coupled with benthic ammonium release (0.32 mg/L), intensify bottom oxygen consumption. Furthermore, the channelized morphology near the dam (width-to-depth ratio 41.7) interacts with sluggish circulation to expand the hypoxic zone via horizontal advection, resulting in localized, year-round hypoxia. This study clarifies the DO evolution pathway under multi-mechanism synergy, providing a mechanistic basis for hypoxia prevention and ecological management in similar reservoirs.