Abstract: Emulsion is widely used in mechanical processing equipment. In order to illustrate the migration characteristics and lubrication mechanism of the emulsion under hydrodynamic lubrication, fluorescent agents R6G and Nile Red with purity of 99% and 98% respectively were added to the trial-produced and commercial emulsions at a concentration of 0.5 mmol/L. The treated emulsion was diluted according to a 10% concentration gradient. The interference method was used to measure film thickness of emulsion under hydrodynamic lubrication while the fluorescence technique was adopted to measure the three-dimensional distribution of pool at the periphery of the contact area and the migration behavior of emulsion droplets. The intrinsic parameters of emulsion (such as viscosity, refractive index characteristic peaks of infrared absorption spectrum and the particle size of emulsions) were also characterized to analyze the emulsion lubrication mechanism under hydrodynamic lubrication. The results showed that the viscosity first increased and then decreased, and a flow pattern transition from water-in-oil to oil-in-water was observed during the dilution process of the emulsion. The emulsions before and after the transformation had good film-forming ability which was positively related to the viscosity, but the film thickness crossed when the concentration was small at different entrainment speed. With the increase of velocity, the universal migration law of emulsion pools was outlet cavitation first and then inlet meniscus with different concentrations. However, the entrainment speed required for the outlet cavitation and the inlet meniscus increased as the concentration of the emulsion decreased. When the water content reached a certain critical value (the concentration of the emulsion in the test reached about 5%), the vapor pockets formed at the exit were difficult to observe and the shape of the emulsion pool changed greatly. Under the condition of sufficient supply of emulsion but less content during the transportation process, the film thickness still reached more than 1 um. The lubrication mechanism of emulsion under hydrodynamic lubricatio was related to particle size and the smaller droplet size can pass through the contact area completely. With the increase of entrainment speed, the film thickness increased and the larger droplets can enter the contact area. The size of the emulsion droplets was not uniform, which suggested the film thickness crossed at a lower concentration. When the droplets size was close to or larger than the convergence gap, the oil in water broke inside or before the contact area under the combined action of the interaction force between the droplets, the shear force and the sliding force. After the rupture, there was a competitive relationship between the oil and water phases. Theoretical analysis showed that the oil phase formed an oil film in the contact area by plate-out mechanism and played a major load-bearing role.