Abstract: This paper gives the relations between the changes of physicochemical properties of crude oil and the solution oxygen and trace elements in injected water in the course of development of Daqing oil field by water flooding.The contents of solution oxygen and trace elements in the source of injected water,the factors causing oxidation of crude oil,the catalysis of trace elements in the oxidation reaction of crude oil and their effects on the physicochemical properties of hydrocarbon: in the oil and the methods of studying these factors are briefly discusted.Results of study indicate that the effect of solution oxygen and trace elements in the injected water on the physicochemical properties of crude oil is expressed by the fact that the average molecular weight of crude oil and its contents of carbonyl,cycloal-kane acid and bitumen tend to increase with the increase of area covered by flood.