Abstract: The evaluation of the uncertainty of accelerator industrial CT has always been one of the difficulties in the research and application of industrial CT. In order to evaluate the uncertainty of linear dimension measurement of accelerator industrial CT, an industrial CT size measurement model was established, and the main sources of uncertainty in the measurement were analyzed. Based on the Guide to the Expression of Uncertainty in Measurement (GUM) method, the evaluation of the uncertainty of industrial CT linear dimension was studied. Taking 6 MeV high-energy industrial CT system size measurement as an example, the main uncertainty components of linear dimension measurement of length samples were analyzed, and the uncertainty of the dimension measurement was evaluated. The result is an extended uncertainty of 0.09 mm with an inclusion probability of 0.99, which reflects the accuracy and reliability of industrial CT dimensional measurements. This paper provides a reference for the reliability of the size measurement results of industrial CT.