The invention relates to a foam agent used in carbon-dioxide flooding and a preparation method of the foam agent. The foam agent comprises the following components in percentage by mass: 10%-35% of a foaming agent, 5%-15% of a foam stabilizer and 50%-85% of water. The preparation method comprises the following steps: according to the components and percentage by mass, dividing quantitative water into two parts, adding the foam stabilizer into one part of water and dissolving under the environment of 85 DEG C-90 DEG C to obtain a first liquid; adding the foaming agent into the other part of water under the environment of 30 DEG C-40 DEG C, stirring and dissolving to obtain a second liquid; and mixing the first liquid and the second liquid and fully and uniformly stirring to obtain the finished product, namely, the foam agent used in carbon-dioxide flooding. The foam agent has strong foaming capability and foam-stabilizing capability, the property of the foam agent is less influenced by the temperature; the foam agent and the liquid carbon dioxide are alternately injected into oil reservoir, after the foam agent is foamed, high-permeability large-pore channels are blocked, the gas channeling reservoir of a gas injection well is effectively blocked and the oil-water interfacial tension can be decreased.