To explore the influence of homogeneity in sample preparation on the test dispersion,As detecting in pakchoi by atomic fluorescence method was studied.Data from a group of samples reserved or deleted by the process of quartering method.The differences of homogeneity between the samples were analyzed by the one-factor variable method.The result of uncertainty magnitude was of 10-2.The relative uncertainty of the differences is the same as the uncertainty of dominant concentration curve,percentage of recovery and method bias uncertainty.Hence,the results show that,the uncertainty of homogeneity sample mainly generates the measurement uncertainty of heterogeneity food samples.The influence of homogeneity coming from sample preparation cannot be ignored in the evaluating of uncertainty of heterogeneity food samples.