Abstract: High concentrate heavy water was used as moderator or coolant in a reactor, and its concentration directly effects the safety and performance of the reactor. The samples were generally taken from the process system manually and then analyzed with infra-red spectrophotometry in the laboratory off-line. This study was carried out to quantify high concentrate heavy water of process system accurately, quickly and safely. In this work, infrared absorption spectrum of different concentrations of standard heavy water were gotten on the Fourier infra-red spectrometer off-line, with the single beam spectrum of empty optical path as the background spectrum. A working curve between the absorbance at the wavenumber of 3 410 cm^-1 and concentrations of heavy water was established with linear function. On this basis, the program of high concentration heavy water on-line measurement was designed. The measurement method is accurate, rapid, non-destructive and without irradiation. For 99.85% heavy water concentration, the relative standard deviation was better than 0.01%. When the deter mined range of heavy water was 99.06%-99.98%, the linear correlation coefficient R² was 0.999 9. This method could also be extended to determine heavy water with other concentration ranges.