The invention relates to a method for preparing high purity oxygen. The method has the characteristics of high purity oxygen product yield, low investment, and lower cost. The technical scheme of the invention is as follows: the method for preparing high purity oxygen by an oxygen-nitrogen liquefying apparatus, comprises following operation processes of: a, introducing nitrogen gas output by the oxygen-nitrogen liquefying apparatus to an absorption cavity of a first rectifying tower evaporator for absorbing cold and transforming the nitrogen gas into liquid nitrogen, and then conveying the liquid nitrogen back to the oxygen-nitrogen liquefying apparatus; b, conveying the liquid nitrogen output by the oxygen-nitrogen liquefying apparatus to a second rectifying tower condenser for releasing cold and transforming the liquid nitrogen into nitrogen gas, and then conveying the nitrogen gas back to the oxygen-nitrogen liquefying apparatus; and c, conveying the liquid oxygen output by the oxygen-nitrogen liquefying apparatus to the first rectifying tower for releasing cold, discharging the components having evaporating temperature higher than oxygen in a liquid form, conveying other vaporizing gases to an absorption cavity of the second rectifying tower condenser for absorbing cold, discharging the components having evaporating temperature lower than oxygen to atmosphere in a gaseous form, conveying other components to the second rectifying tower for further rectifying, and outputting the obtained liquid high purity oxygen product.