Abstract: Heavy water can be used as moderator material of a nuclear reactor. The concentration of heavy water affects the safety and property of the reactor directly. This work was carried out in order to research the type of relationship curve between heavy water concentration and absorbance. Based on Lambert Beer’s law, the formulas between heavy water concentration and absorbance were theoretically derived respectively when the absorption peak of a certain kind of water molecules in liquid heavy water overlaps with the absorption peak of zero to two other kinds of water molecules. The types of relationship curve between heavy water concentration and absorbance were discussed in different cases. The absorption spectra of heavy water in the concentration range from 0.015% to 99.98% (mol vs mol) were recorded by fourier transform infrared spectrometer and the types of relationship curve were analyzed. It showed that the theory tallied with the practice. The type of relationship curve between heavy water concentration and absorbance was quadratic curved in a wide concentration range and linear when the concentration range was narrow. In special cases, the absorbance was fixed or the type of relationship curve was linear.