In order to determine the reliability standard of the result of measuring the phosphorus in maize grain by spectrophotometry with phosphorus molybdenum blue, uncertainty sources of this method was identified and analyzed according to the requirement and method of GB 5009.87-2016 Nation Food Safety Standards: Determination of Phosphorus in Foods and JJF1059.1-2012 Guidance on Evaluating the Uncertainty in Chemical Analysis. Each uncertainty component was synthesized and extended. The results showed that the preparation of sample and preparation of standard solution were larger contribution to the uncertainty of synthesis. In the process of determination,enough attention should be paid to improve the test method and reduce the uncertainty. The results provide references for the evaluation on uncertainty of measuring the phosphorus in other agricultural products by spectrophotometry with phosphorus molybdenum blue.