In order to solve the problem that the film on the contact interface surface of the carbon brush/slip ring assembly of the hydro-generator set is difficult to form and easy to be damaged. Similar tests are carried out on a current-carrying friction and wear test machine with the same size and structure as the one in service, to explore the mechanism and key factors working on the formation and destroy of film on the surface of carbon brush and slip ring. Based on the analysis of the causes of the film on the surface of the current-carrying friction pairs, the effects of current density, velocity, temperature, and load on the surface film formation of the collecting ring were studied. The surface square resistance of the slip ring was detected with a four-probe square resistance tester to understand the formation of the surface film layer without damaging the collecting ring. The maximum square resistance of the positive electrode surface could reach 0.6 mΩ at 4 m/s and 8 m/s, and the maximum square resistance of the positive electrode surface can reach 25 mΩ at 12 m/s, and the high rotational speed tearing the surface carbon film promoted contact of the base metal with oxygen. The current increased the temperature of the friction pair, which was conducive to the oxidation reaction, but too high temperature would lead to the desorption of water molecules on the friction surface, resulting in serious wear and damage to the surface film. The temperature of the negative carbon brush was generally higher than that of the positive carbon brush. Uneven spring pressure would lead to an uneven current carrying the carbon brush, which was more likely to cause high-temperature damage to the surface film of the negative carbon brush. Film generation was the fastest at 12 m/s and 800 A current. It took about 200 minutes for the topography of the carbon brush and the collector ring to be run in, after which the surface film began to form gradually. The surface film was mainly composed of a graphite layer and an oxide layer. The size of the film resistance was mainly affected by the film thickness. During the formation of the surface film, the contact resistance between the carbon brush and the collector ring is mainly based on shrinkage resistance in the early stage, and film resistance in the later stage. The rotational speed is the main factor affecting the rate of surface film formation. High temperature is the direct cause of damage to the surface film. In this paper, the current-carrying operation of the carbon brush/slip ring under actual working conditions is restored to the greatest extent, and the formation mechanism of the surface film is analyzed, which has a certain guiding significance for the formation and maintenance of the surface film of the collector ring.