Abstract: Water lubrication has became immensely popular due to its environmental friendliness and low cost. However, the challenge lies in achieving an effective lubrication film with water's low viscosity. Usually water-lubricated bearings could suffer from wear and noise under tough working conditions, such as low running speeds at start up/shut down/dynamic positioning, and heavy dynamic loads when subjected to waves. In fact, new bearing structure and new bearing materials have been studied to protect the water-lubricated bearings under the tough cases. It is certainly good to find alternative solutions beyond the existing schemes. The author's research group designed a new way for water lubrication assisted by a small amount of secondary lubricant, and it was found that a short-term supply of a small amount of emulsifying oils or silicone oils was conducive to reducing friction and wear of water-lubricated bearings. In order to further explore the mechanism of lubrication with a small amount of lubricant under water environment, a roller-on-disc lubrication film test rig was used to directly measure the film formation behaviour when a small quantity of oil was injected. Two environmentally friendly lubricants, Bio-oil A of low viscosity and Bio-oil B of high viscosity were provided by a micropette (1 μL/s, total 10 s) under the load of 2 N at various speeds. The film thickness in the contact area after a short-term supply of lubricant was recorded, and therefore the assistant lubrication characteristics of two lubricants as the second medium in the water environment were revealed. Moreover the fluorescent approach was used to observe oil flow. The results showed that the water lubrication assisted by a small amount of lubricant was beneficial to improve the film-forming capacity of the contact area, and the lubricant supply of the roller-disc contact area mainly came from the inlet oil pool and the side-oil ridges. Wettability of the oil on the disc surface decreased in the water environment compared to those in air, making the oil supply susceptible to external interference and leading to fluctuation in film thickness. The instability of the side oil- ridge and the inlet pool was the main reason for the fluctuation of film thickness. At lower disc-speeds, the side oil-ridge was more likely to flow back to the lubrication track, which could improve the oil supply, and it was showed that the oil with lower viscosity demonstrated more reflow. At higher disc-speeds, the oil was pushed away at the inlet area by both mechanical diversion from the contact and water adverse flow, resulting in a poor inlet oil-supply. It was more difficult for Bio-oil B with higher viscosity to be replenished into the contact area than Bio-oil A with lower viscosity. In addition, the results of the block-on-ring tribometer showed that the water lubrication assisted with a small amount of lubricant presented obvious reduction of friction and wear. The higher the speed, the greater the influence of reverse flow. Bio-oil B, which had a higher viscosity had a better friction reduction effect.