Karl Fischer coulometry is a classical method for the determination of water content. It has a potential in the determination of water content in gas owing to its high sensitivity and accuracy. In present study, a comparison method was employed to investigate the accuracy of Karl Fischer method for water content in gas, namely the gas with a stable humidity produced by a humidity generator was provided to the dew-point meter and Karl Fischer coulometer simultaneously. The effects of initial drift and sample mass on the repeatability of water content were investigated. It indicated that the lower initial drift and larger sample mass make more repeatable results. The effects of methanol content in the Karl Fischer reagent on the reaction were investigated. It indicated that the lower the methanol content, the greater the indication error. The accuracy of Karl Fischer coulometry for the gas with a dew-point in the range of (-60℃~-16℃) was investigated. It indicated that the error of Karl Fischer coulometry (0.4℃~2.6℃) was small with a dew-point of (-50.5℃~-19.9℃). However, the error Karl Fischer coulometry was -5.9℃ with a dew-point of -58.1℃. These results were valuable reference for the application of Karl Fischer coulometry for the water content in gas.