Study region The Dai Lake (2305 km(2)) located in Ulanqab City of Inner Mongolia's semi-arid region, representing typical inland lakes under severe water stress globally. Study focus This study integrates the Budyko framework with policy assessment to quantify climate change and human activity contributions to hydrological evolution (1970-2023). Using Mann-Kendall trend analysis, mutation detection, and elasticity coefficient methods, we identified 2002 as a critical transition point and analyzed meteorological variables, land use changes, and socio-economic factors impacting runoff processes while evaluating recent water management policies. New hydrological insights for the region The closed basin experienced dramatic changes since the 1970s: lake area decreased from 164.11 to 45.46 km(2) with 12.28 m water level decline. Human activities dominated hydrological changes (-84.24 %) versus climate impacts (-15.76 %). The runoff-precipitation relationship changed fundamentally after 2002, with slope decreasing from 0.12 to 0.04, indicating weakened response to intensive water consumption. Agricultural water use was the strongest negative driver (r = -0.84, p < 0.01). Policy interventions since 2014 showed measurable effectiveness, reducing livestock from 730,000 to 320,000 and shifting human contribution to positive (91.92 %) in 2020-2023. Despite improvements, water structure remains imbalanced with 85.08 % groundwater dependence and only 7.26 % ecological allocation, highlighting needs for sustained policy implementation in semi-arid inland lake systems.