The present invention provides a concrete protective agent and a preparation method thereof, and a concrete protective film and a preparation method thereof. The concrete protective agent provided in the present invention includes the following components: water, oxalic acid, a defoaming agent, and a film-forming agent. When the concrete protective agent provided in the present invention is used for concrete protection, oxalic acid in the protective agent can react with calcium ions in concrete for in situ generation of calcium oxalate monohydrate inside and on a surface of concrete to obtain a protective film with strong adhesion to concrete. The film-forming agent in the protective agent is used as a template to adjust and control growth of calcium oxalate crystals, so as to improve waterproof performance and corrosion resistance to sulfate and chloride ions of the protective film. Preparation methods provided in the present invention are simple and practical and are suitable for mass production.