Precise determining method for small-amount uranium

The invention discloses a precise determining method for small-amount uranium. The precise determining method includes weighing a uranium sample, preprocessing, adjusting valence of the uranium sample to obtain uranium (IV), then adding excess potassium dichromate, which is 0.3-0.8% of the total amount of the potassium dichromate, to oxidize U (IV), then adding an organic solvent to extract excess chromium (IV), and determining the chromium in organic phase by the spectrophotometric method. The adding method of the potassium dichromate includes preparing and mixing two kinds of potassium dichromate solution with different quality concentrations, and then adding in one step. When the sample amount is less than 50mg, precision and accuracy of the small-amount uranium precise determining method is improved, determining precision and accuracy are both higher than 0.05%, and determining results are accurate and reliable.