Oil slick pollution seriously influences the ecological environment. Many countries have invested a lot of money and workforce in investigating oil slick on water. Thickness measurement of oil film on water with high precision is important for the prevention and treatment of oil slicks. Here,diesel was taken as the research object. Fourier Transform Infrared Spectrometer obtained theabsorption spectra of diesel at different temperatures (293/298/303/308/313/318 K) and water at room temperature (298 K). It was found that diesel and water were absorbed in the near-infrared region (6 000.0~11 000.0 cm-1),and the absorption spectra of diesel did not shift with temperatures. The wavenumberν1 (8 381.6 cm-1) with the maximum absorption coefficient of diesel was selected to establish the inversion model of oil film thickness on water based on a single-wavelength. ν1 and the wavenumber ν2 (8 918.7 cm-1) with an absorption coefficient around zero were selected to establish an inversion model of oil film thickness on water based on dual-wavelength. Furthermore,the absorption coefficients of water at these two wavenumbers were very small and had little influence on the measurement results. A calibration tool with known oil film thicknesses (0~1 000 μm) was employed to validate the measurement precision of the inversion models. It was found that the average relative deviations between the film thicknesses obtained by the two models and the known values were 36.4% and 2.5%,respectively. The maximum relative deviations were 44.7% and 3.7%,respectively. The maximum standard deviations were 7.0 and 5.6 μm,respectively. It revealed that the dual-wavelength model was superior to the single-wavelength model. On this basis,a novel film thickness measurement system for oil on water with dual-wavelength absorption spectroscopy was developed. The temporal resolution of the developed system was 0.03 s. The variations of oil film thicknesses after dropping a certain volume (1 mL) of diesel on the water surface were investigated,and the ultrasonic pulse echo-method was simultaneously employed to compare with the developed system. Each drop of diesel was measured 10 times by the two methods. A total of 20 groups of oil film thickness data (16~35 mL) were measured,and the average values of the results measured by the two methods were compared. It showed that the average relative deviation of the oil film thicknesses measured by the two methods was 2.5%. Moreover,the maximum relative deviation was 3.7%. The maximum standard deviation of oil film thicknesses obtained by 10 repeated measurements of dual-wavelength absorption spectroscopy was 6.4 μm. In summary,the measurements of the thickness of oil film on water could be achieved by the developed high-precision system. It had the advantages of interference-free,fast-response,compact-structures,etc.,and could be applied to different types of oil slick measurement. The developed system was expected to provide new ideas and scientific guidance for the monitoring,preventing and treating oil spills.