Abstract: A carbon-based solid acid catalyst, a preparation method of the catalyst, and a method to use the catalyst for hydrothermal conversion of biomass are provided. The preparation method of the carbon-based solid acid catalyst includes the following steps: S1. mixing pectin with water, adding concentrated sulfuric acid for activation, and adding a resulting mixture to an ionic resin with an aromatic ring matrix; S2. drying a material obtained in S1, crushing a dried material into a powder, and subjecting the powder to pyrolysis in a dry inert gas; S3. subjecting a solid obtained after the pyrolysis to sulfonation with concentrated sulfuric acid; S4. diluting a material obtained in S3 with water, filtering a resulting mixture, and washing a resulting filter residue with water until no sulfate ions are detected in washing water; S5. drying the filter residue.