A temperature-resistant salt-tolerance nano emulsion oil-displacing system used in tertiary oil recovery is characterized by comprising following components, by mass: 0.01-0.5% of an alkane as a dispersed phase, 5-25% of a non-ionic surfactant, 5-20% of a zwitterionic surfactant, 10-20% of a cationic surfactant, 10-35% of a low-carbon alcohol as a cosurfactant and the balanced water. The nano emulsion oil-displacing system is prepared through following steps: (1) uniformly stirring water and the surfactants together at 5-40 DEG C through a magnetic stirrer at a stirring rate being 100-500 rpm; (2) adding the low-carbon alcohol as the cosurfactant; and (3) adding dropwisely the dispersed phase to the system after the system being stirred uniformly, and meanwhile stirring the system at a stirring rate being 100-500 rpm for 5-60 min to obtain the nano emulsion. The nano emulsion is less than 100 nm in average particle diameter and is excellent in a temperature-resistant salt-tolerance performance. An indoor oil-displacing test result proves that by means of 0.1% of the nano emulsion, recovery efficiency can be increased by 7.9% on a basis of water drive; and by means of 1% of the nano emulsion, the recovery efficiency can be increased by 19.8% on a basis of water drive.