Natural drylands often exhibit multistability-distinct ecosystem configurations sustained under similar climates by internal feedbacks. Here, we test whether such multistability persists in human-managed drylands and evaluate how water management stabilizes ecosystem states. Focusing on China's pan-Hexi Corridor-a representative oasis-desert mosaic spanning a long history of water reallocation-we integrate multi-source datasets to quantify an ecosystem-quality index (EQI). Combining natural water resources with composite human-activity intensity, we use potential-landscape and resilience diagnostics to identify multistability and apply regression models to quantify drivers and generate projections. Potential-landscape analysis reveals two natural low-EQI states coexisting, while a high-EQI state emerges where water and human-activity intensity are sufficient. Overall, human activity raises EQI but reduces resilience. Sectorally, forestry and environmental water are positively associated with EQI, whereas aquaculture and domestic uses are negative. Under CMIP6 scenarios, projections indicate regional EQI declines over the next 50 years, with 0.69-0.91% of landscapes at risk of detrimental regime shifts; maintaining current quality would require increasing forestry water by 82-337% or environmental water by 83-341% across scenarios. Our findings underscore that sustainable water management is pivotal for preventing abrupt degradation while balancing ecological benefits with resource constraints.