The invention discloses a preparation method of a lignin based carboxylic acid derivative. The preparation method includes: taking lignin as the raw material, firstly mixing lignin with acid uniformly to obtain a lignin acid solution, and then adding a nitrite solution rapidly into the lignin acid solution dropwise to carry out oxidation reaction, thus obtaining the lignin based carboxylic acid derivative. The reaction process of the method provided by the invention is carried out under atmospheric pressure and low temperature, the raw material cost is low, the technology is simple and is easy to operate and control. Without destroying the lignin benzene ring structure, the method provided by the invention oxidizes the hydroxyl carboxyl group on a lignin side chain structure to generate carboxyl, and has the advantages of large carboxyl loading amount and not obvious molecular weight degradation.