Abstract: A device with high temperature Cr inside was made to reduce water to hydrogen. The method for reducing the interferer of H3+ to HD+ was discussed. The two of the set of standard water samples which have natural level ratio of D and H as standards and the other two as unknown samples, a linear calibration method with equal H2+ was studied on GAM400 quadrupole mass spectrometer. And the difference calibrated coefficient of the ratio of D and H was obtained between two standards. The other two standard water samples was measured based on the methods and the coefficient. The measured results has ±0.1% bias to standard values. And the evaluated relative uncertainty is 0.8％.