The oil spill volume of the oil-in-water emulsion is an important indicator for evaluating and analysing oil spill pollution on the sea surface. Laser-Induced Fluorescence (LIF) is currently one of the most distinguished technologies in remote sensing detection of oil spills on the sea. Currently, there is no effective and complete algorithm for assessing the oil spill volume of oil-in-water emulsion on the sea surface based on LIF detection technology. In view of this, firstly, an equivalent evaluation model of the oil spill volume of oil-in-water emulsion is designed: the dispersed phase oil droplets in the oil-in-water emulsion are assumed to gather into a piece of oil film floating on the sea surface, so the evaluation of the oil spill volume of oil-in-water emulsion is transformed into the estimation of oil film thickness in the equivalent model with the same fluorescence effect; Secondly, based on LIF detection mechanism and fluorescence radiation transmission process, the equation of fluorescence signal received by LIF system is established, furthermore, the calculation formula of oil film thickness is obtained by inversion; Finally, two representative heavy and light oils are selected, the correctness of the equivalent model is verified by simulation experiments, simultaneously, the fluorescence signal intensity of the detected oil-in-water emulsion was calculated by the equivalent algorithm to obtain the oil film thickness and the error analysis was carried out, and then the applicable conditions of the equivalent evaluation algorithm are obtained: that is, when the actual oil spill thickness of oil-in-water emulsion is less than or equal to the minimum oil spill thickness when its fluorescence is stable, the equivalent evaluation algorithm in this paper has high accuracy, and its average error is less than 8%; Nevertheless when the actual oil spill thickness is greater than the minimum oil spill thickness when the fluorescence is stable, the equivalent evaluation error increases, and the average value exceeds 25%. Meanwhile, when using the algorithm in this paper to evaluate the oil spill volume of heavy and light oil-in-water emulsions, better estimation results can be acquired when the actual oil spill thickness is not greater than 2 and 16 μm, respectively. Therefore, the research content of this paper is a feasible method to evaluate the oil spill volume of oil-in-water emulsion on the sea surface based on LIF technology.