Abstract: The isotopic composition of hydrogen and oxygen in heavy water are analyzed by using a high-resolution mass spectrometer.The molecular forms and single-charged ions in the mass spectrum of heavy water are clarified.And the hydrogen and oxygen isotopic compositions in some heavy waters are calculated by the high-resolution mass spectrometer.The results show that hydrogen and oxygen isotopic composition can be analyzed in the same time by using high-resolution mass spectrometer,little sample is used and the results is reliable.