Dissolved oxygen (DO) is an important indicator of aquatic ecosystem health.The primary production of a lake can be reflected through DO.Study of DO and its vertical distribution is important to understand the eutrophication process of lakes and reservoirs.A three-dimensional hydrodynamic-water quality model (ELCOM-CAEDYM) developed by Center for Water Research of University of Western Australia was applied in order to provide substantial information about vertical distribution of DO at Tianmuhu Reservoir.The simulation duration is from January 1st to December 31st,2011 (365 d).The simulation results of DO,water temperature and total phosphorus (TP) showed good agreement (RMSE<1.8℃,1.8 mg/L and 0.003 mg/L respectively, NSE > 0.7,RE > 10%) with observed data,which indicated that seasonal variation of vertical distribution of dissolved oxygen changed together with thermocline.In winter water column fully mixed at Tianmuhu Reservoir.Thermal stratification started in April and the bottom area became lack of oxygen.Thermocline depth was during 4-10 m below surface with hypoxia (DO<2 mg/L) in the bottom water with an area of 35% of whole bottom area in June.When thermocline disappeared in autumn the vertically-averaged DO concentration started to increase and the water column tended to mix again.DO and TP concentration were influenced by inflows in the riverine zone while lacustrine zone was dominated by significant release resulting from hypoxic events.The vertical distribution pattern of DO induced by thermal stratification in the reservoir could further affect the water environment through algae activities.