The content of copper element in beryllium is measured by atomic absorption spectroscopy, the main sources that affect the uncertainty of measurement are analyzed, by establishing a mathematical model of each component in the measurement process. Uncertainty components including the sample weighing, dissolution, extraction, preparation of standard solution, work curve fitting and sample repeat determination are analyzed, the expanded uncertainty is got. The results show that the sample repeat determination and work curve are the main factors affecting the uncertainty of the method, which provides a theoretical basis for the determination quality control of copper content in beryllium by atomic absorption spectroscopy method.