The invention provides high-strength polymer nanoribbons and a preparation method thereof. The nanoribbons are spontaneously generated in a polycondensation forming process of a polymer, wherein the polymer has the structure general formula as described in the specification, wherein R represents NH2, OH, alkoxy or alkyl, alkoxy is selected from methoxyl and ethyoxyl, and alkyl is selected from methyl or ethyl. The whole-carbon ladder polymer is formed by polycondensation of two monomers and spontaneously generates the nanoribbons during the forming process, no template agent and no subsequent processing are required, and a new technical idea and a new solving method are provided for synthesis of the high-strength nanoribbons; and the prepared polymer nanoribbons have high strength and excellent mechanical properties, and can be well used as reinforcing materials in composite materials.