The present invention belongs to the technical field of biomass carbon materials, and relates to a lignin porous carbon nanosheet, a preparation method therefor, and an application thereof in supercapacitor electrode materials. The method of the present invention performs layer-by-layer self-assembly of sulfonated lignin and oxalate in a selective solvent to prepare a layer-by-layer self-assembled lignin/oxalate composite, which is then carbonized and pickled to obtain the lignin porous carbon nanosheets. The lignin porous carbon nanosheets prepared by the above method of the present invention have a specific surface area of 200-1500 m2/g, a micropore specific surface area of 100-500 m2/g, a mesoporous specific surface area of 100-1000 m2/g, a pore diameter of 0.5-30 nm, and a pore volume of 0.5-1.5 cm3/g; they can be applied to supercapacitor electrode materials, showing higher specific capacitance and excellent rate performance (with a specific capacitance retention rate of 76.6%), having good potential application value.