Flame atomic absorption spectrophotometry is one of the most important methods for the analysis and detection of the concentration of metal elements.The principle of determination of Pb and Cd in heavy metal waste water by atomic ab-sorption spectrophotometer is introduced,and the influence of Na content in waste water on the results of determination of Pb and Cd is emphasized.The control range of Na content and concentration in determination of Pb and Cd by flame atomic absorption spectrophotometer is studied through comparative experiment,so as to understand the interference degree of Na content on this method.