Abstract: The trace hydrogen in the natural gas from the mantle-derived fluid areas is the best specimen to reveal the isotopic composition characters of mantle-derived fluids. Restricted by the present measurement methods, the detection limit of hydrogen for accurate measurements of hydrogen isotopic composition should be further improved. It was proved that the measurement error of hydrogen isotopes was relatively larger and reliability of the measurement results deteriorated when the hydrogen content was lower than 1.5%, which may relate to the random fractionation of hydrogen isotopes around the inlet needle when it was sampled by the carrier gas. In order to avoid this random fractionation, an improved pressure sampling method was brought out, that is, increasing the pressure inside the sample container by pumping in salt-saturated water. The experiment results showed that by this sampling method the range and the standard deviation of the measured hydrogen isotope values declined with the increasing of the hydrogen content within 1% range and the absolute error was equivalent to that of the system error of MAT 253 instrument. Thus, this sampling method could effectively avoid the random fractionation of hydrogen isotopes during the sampling and improve the accuracy and reliability of the measurement results. It was also proved that the presently employed machine could be reasonably improved in order to enhance its function.