Taking a reservoir in Guangdong Province as a case study, the relationship between contents of bottom dissolved oxygen(BDO)and bottom temperature(BT), pH, conductivity, turbidity, total phosphorus(TP), total nitrogen(TN), chemistry oxygen demand(COD), total organic carbon(TOC), chlorophyll-a in water were analyzed according to the monitoring data from April 2013 to March 2014, to state the spatial-temporal distribution characteristics of bottom dissolved oxygen and the mechanism of hypoxia. The results indicated that, BDO concentrations are lower in spring and summer, while higher in autumn and winter(dry seasons). BDO concentrations are higher in the water intake and lower in the middle of reservoir. Hypoxia mainly happens in summer(form May to August). The correlation analysis showed that the relationship between the contents of BDO(P<0. 05)and the contents of BT, turbidity, TOC and chlorophyll a was positive, which was negative with the contents of the water depth. The positive correlation between the contents of BDO and chlorophyll a and that dominate species are cyanobacteria indicated that the vertical migration of cyanobacteria is one of the key factors affecting BDO. Therefore, the temporal change of the contents of BDO was affected by BT; the spatial change of the contents of BDO was affected by hydrodynamic force; hypoxia might be affected by the thermal stratification.