In order to analyze the specific situation of the GUM method and MCM in evaluating the uncertainty of bearing measurement and explore the difference between them, taking the evaluation of the roundness error of the outer ring of the bearing as an example, the output results of the GUM method and MCM and the variation of 95% probability interval were observed by changing the input distribution type, and the verification for the GUM method was carried out. The results show that when the uncertainty of bearing roundness error is evaluated by using the GUM method, the reliability of the results obtained by the input distribution of normal distribution is higher;when the input distribution type is non-normal distribution, the standard uncertainty of GUM method is 0.427 0 μm, which is significantly higher than that of MCM (0.315 6, 0.413 4, 0.213 6 μm);when MCM is used, the output of MCM will be adjusted according to the change of input distribution type, and it is adaptable, no matter whether the input is normal or not, accurate results can be obtained, and it does not need complicated derivation calculation, it is very operational.