Abstract: By reducing the fuel oxygen uranium ratio of uranium dioxide (UO₂) pellets and increasing its high temperature compatibility with the cladding, the fuel burn up demand of the pellets can be met. The uranium oxide atomic ratio test method of the UO₂ pellets in this study is oxidized into stable uranium trioxide in an aerobic atmosphere by using a thermogravimetric analyzer. The oxygen uranium ratio of the UO₂ pellets is obtained by accurately measuring the net weight of the sample after dehydration and the total weight after oxidation. Different from ISO 12795-2004 and GB/T 11842—1989, the effects of the small sample number, different heating temperatures and different oxidation atmospheres on the oxygen uranium ratio of UO₂ are studied in this work. The source of uncertainty in the test of oxygen uranium ratio is analyzed and the uncertainty evaluation carried out. By comparing the results of oxygen-uraniam ratio tests in different laboratories, the stability of the thermogravimetric analyzer for measuring the oxygen uranium ratio is verified, and the stability of oxygen uranium ratio detection is evaluated by using internal standard samples. This study provides data support for the application research of the UO₂ nuclear fuel.