The invention provides a self-assembled ultramicro-foam oil-displacing agent. The self-assembled ultramicro-foam oil-displacing agent contains the following components in percentage by weight: 0.2%-0.3% of 2[N-methylene-N-dimethyl(1-dodecyl-1-sodium carboxylate)-methine]ammonium bromide, 0.2%-0.3% of N-(1-dodecyloxy ether-2-hydroxy)propane-N-dimethyl-N-(2-hydroxy)propanesulfonate, 0.1%-0.35% of an anionic surfactant, 0.02%-0.07% of triethanolamine, 0.02%-0.07% of urea, 0.04%-0.14% of a chelating agent, 5%-7% of inorganic salt and the balance of water. The invention further provides a preparation method and application of the oil-displacing agent. The self-assembled ultramicro-foam oil-displacing agent has the beneficial effects that rich and fine foams are produced, the diameter are 1-20 microns, and the half-life period of the foams is relatively long; and when CO2 is introduced for foaming, the phenomenon that CO2 generate chemical reaction with calcium ions and magnesium ions to generate white precipitated calcium carbonate or magnesium hydroxide is avoided.