Abstract: In order to analyze the relationship between the physiology of Nymphaea tetragona and the purification,Nymphaea tetragona was cultivated in 9 tanks with various planting densities to investigate diurnal variation of physiological characteristics of plants including peroxidase(POD) and catalase(CAT) activities of the root tissues,soluble proteins(SP),chlorophyll(Chl) mass concentrations of the leaves and main physical and chemical characteristics of the effluents.Meanwhile,morphological dimensions,tillers,root densities,biomass and nitrogen(N) and phosphorus(P) accumulation of plants were measured.Correlation between diurnal variation of physiological characteristics of plants and N and P removals of effluents was analyzed,and effects of planting densities on physiological characteristics of the plants and purifying effects of the effluents were studied,and a tank without planting for control(CK) was set.Resultsshowed POD and CAT activities of the root tissues,Chla,Chb and SP mass concentrations of the leaves had positive correlations with NH₄⁺-N and soluble P removals of the effluents.In the tanks with higher planting densities,the plants had longer roots,stems and leaves,more tillers,root densities,biomass and N and P accumulations,higher Chla,Chb and SP mass concentrations of the leaves and NH₄⁺-N and soluble P removals of the effluents,while POD and CAT activities of the roots tissues declined.With stronger sunlight,POD and CAT activities of the roots tissues,Chla,Chb and SP mass concentrations of the leaves,NH₄⁺-N and soluble P removals of the effluents all increased.Chla,Chlb and SP mass concentrations of the leaves and DO concentrations of the effluents showed double-peak styles at about 12:00 and 14:00,and the bottom between two peaks appeared at about 13:00.At 10:00-14:00,CAT and POD activities of the root tissues,Chla,Chlb and SP mass concentrations of the leaves,NH₄⁺-N and soluble P removals of the effluents all exceeded the other times at the day.