The uncertainty of the determination of phytic acid content in soybean by spectrophotometry was evaluated in the paper. A mathematical model was established and source of measurement uncertainty discussed. According to the requirement and method of Evaluation and Expression of Uncertainty in Measurement（JJF1059-2012）,Guidance on Evaluating the Uncertainty in Chemical Analysis,the component uncertainties were synthesized and extended. The results showed that method recovery standard,solution preparation,fitted curve were the major factors of uncertainty. An expanded uncertainty of 0.957 2 mg/g was obtained at the confidence probability of 95% and coverage factor of 2,the results of determination of phytic acid of soybean was（15.58 ±0.957 2） mg/g.