In order to evaluate the creditability of ash content and find out the main influence factor, test process and calculating style for ash content in carbon fibre is analysed. The uncertainties of various variables and their sources in the process of test are brought forward and calculated. The combined uncertainty and expanded uncertainty are finally obtained. When ash content is 亿0. 1%, the expanded uncertainty is 0. 03% and the uncertainty coming from repeatability of results of measurements is the main influence factor. When ash content is <0. 1%, the expanded uncertainty is 0. 01% and the uncertainty coming from system of measurements is bigger than that coming from repeated measurements and it is the main influence factor.