Abstract: The uncertainty of the determination of iron in pitaya fruit was evaluated by the microwave digestion-flame atomic absorption spectrometry. The corresponding uncertainty mathematical model was established. According to the analysis of uncertainty sources in the test process, there were 7 aspects. The uncertainty components were calculated. The result of the determination of iron content in Pitaya was X=(1.308±0.281) mg/100 g(k=2). It was found that the main sources of uncertainty in the measurement process were the measurement of sample solution, the blank of sample and the constant volume of sample solution. Among them, the uncertainty caused by standard curve making and working curve fitting was the largest. Uncertainty assessment results can provide references for the detection of nutrients, avoid or reduce some unnecessary detection errors, and make the test results more accurate.