Oil film thickness is an important indicator for the assessment and analysis of oil spill pollution on the sea surface. Laser-Induced Fluorescence (LIF) is one of the most effective technologies for oil spill detection at present, the oil film thickness inversion algorithm based on LIF is only suitable for thin oil film (≤10~20 μm), there is no effective method for the evaluation of thicker oil film (>20 μm). In view of this, an inversion algorithm based on LIF detection technology for evaluating thicker oil film is proposed, the algorithm uses oil film fluorescence signal to invert oil film thickness, deduces oil film thickness inversion formula, and gives oil film thickness evaluation method based on the inversion algorithm. First, Otsu algorithm is used to select the appropriate fluorescence spectrum band, and then the oil film thickness is retrieved according to the spectrum data of each wavelength in the selected band, finally, the average value of the retrieved oil film thickness is used as the oil film thickness evaluation result. The applicable range of the algorithm is studied, the relationship between the maximum value of the effective evaluation range of the algorithm and the relative measurement error is given, the maximum value of effective evaluation range of oil with different extinction coefficients under various measurement errors is given. The method in this paper is verified by experiments. The mixture of crude oil and mineral oil (1:50) is selected as the experimental oil and the laser with a wavelength of 405 nm is used as the excitation source. The collection wavelength range is 420~750 nm. The background fluorescence and Raman scattering spectra of sea water, the fluorescence spectra of experimental oil and various thick oil films are collected to invert the oil film thickness. Otsu algorithm is used to select the band of 420~476 nm to evaluate the oil film thickness. The evaluation results show that when the oil film thickness is ≤800 μm, the algorithm has high accuracy, with an average error of 10.5%; when the thickness is >800 μm, the average error is 28.8%, with a large evaluation error and rapidly increases with the increase of the oil film thickness. The analysis results of relative error and extinction coefficient are consistent with the experimental results. The results show that the method can effectively evaluate the thickness of the thick oil film on the sea surface, and judge the effectiveness of the evaluation results according to the measurement relative error and extinction coefficient.