A carbon-based solid acid with high acid density was successfully prepared using camphor tree branches as raw materials through a novel method including dilute sulfuric acid activation, carbonization in refluxing solvent and sulfonation. Physical characterization was detected to show that the carbon-based acid is amorphous with polycyclic aromatic carbon sheets attached plentiful -OH, -COOH, and -SO3H groups. The sulfonic acid density and total acid density of it reached 2.05 mmol·g^-1 and 5.63 mmol·g^-1, respectively by acid-base titration. As a solid acid catalyst, it showed excellent performance in the ketalization of cyclohexanone with glycol.