A method that could be used to quantify the concentration of water in oil for marine-borne machine is discussed in the present paper. Water molecule has distinct absorption bands in mid-infrared section in 3 773~3 509 cm-1, so it could be applied to the quantification of water in oil. An experimental sample cell was designed to accomplish the experiment for the convenience of sample preparation and mount in measurement, and it was designed to suit for FTIR spectrometer. The authors chose to prepare the sample oil by blending different amount of water into oil through titration. The spectra of oil with different water concentration were acquired and their integrals were calculated. The analysis of experimental result shows that the functional relationship between the integral of spectrum in 3 773~3 509 cm-1 and the percentage of water content in oil is a quadratic curve. The relative error between the fitted curve and experimental data is at the level of 0.01%. The research work in this paper presents the possibility of quantification of water content in oil through mid-infrared and gives us an idea to design a kind of new sensor based on mid-infrared LED and PIN for measure and to warn people who monitor the abnormal variation of water content in different kind of oil used in marine-borne mechanical system.