Abstract: The methods of negative interpretation and data processing for the modified standard cylinder test were introduced. The uncertainty origins of test results, mainly from the two aspects of measurement and data processing, were analyzed, and further the uncertainty evaluating method was established. The uncertainties of the modified standard cylinder test in 25mm diameter were evaluated. Results indicate that the expanded uncertainties of the average displacement in radial direction and the expansion time with the degree of confidence about 0.95 are 0.24mm and 0.056μs respectively; the uncertainty of fitting curve is about 0.014mm. Result above proves that the modified standard cylinder test and its data processing have high accuracy.