According to the test method of GB/T 5750.6—2006, the cadmium content in drinking water was measured by graphite furnace atomic absorption spectroscopy, and the accumulated data was statistically analyzed based on the Top-down control chart method. The extended uncertainty was 0.30 μg/L (k=2,P=95%). Using Top-down control chart method to assess the uncertainty is statistically rigorous, and has a wide range of application prospects.