The invention provides a method for preparing lignin carbon-based solid acid. The method is characterized by comprising the following steps: by taking alkali lignin as a main raw material, stirring and dissolving the alkali lignin, alkali and water at room temperature according to a certain mass ratio, adding epoxy chloropropane of a certain mass, stirring for a certain time, obtaining lignin gel at a proper temperature, soaking the gel in an aqueous solution of hydrochloric acid, washing to be neutral, performing vacuum drying, thereby obtaining the dried lignin gel; and carbonizing the dried gel at a certain temperature, sulfonating the gel by using a sulfonating agent, washing to be neutral after sulfonating, thereby obtaining the carbon-based solid acid; or directly sulfonating the dried gel by using the sulfonating agent, thereby obtaining the carbon-based solid acid. The preparation process is simple, the raw materials are cheap and readily available, and resource utilization of industrial waste residues is realized; and the prepared carbon-based solid acid is developed in a porous structure, high in acid amount and high in stability and reusability.