Abstract: The CS samples were randomly measured by different inspectors in the blind sample, at different time, random environment conditions and different equipments, and using the GB/T 13610-2014 Analysis of Natural Gas Composition-Gas Chromatography Method. Through the test of normality, bias, the check standard sample (CS sample) validity and the single value I-MR control chart, the uncertainty by using Top-down method is evaluated under the controlled state of the whole measurement system. It is concluded that the period precision of each component in the measurement is larger than the repeatability of measurement and smaller than the reproducibility required by GB/T 13610-2014, so the uncertainty evaluation is reasonable. The bias uncertainty component can not be ignored according to the uncertainty of the CS sample, which is relatively high.