The invention discloses a high carboxyl content lignin grafted polycarboxylate, a preparation method and application thereof. The preparation method includes: by weight, mixing 10-30 parts of alkenylglycidyl ester, 30-50 parts of alkenyl carboxylic acid, 20-40 parts of alkenyl carboxylate and 100-200 parts of deionized water evenly, and performing heating to 50-80DEG C to obtain a mixed solution;dissolving 0.5-1.5 parts of an initiator in 20-40 parts of deionized water, then adding the mixture into the mixed solution, and carrying out reaction at 50-80DEG C to obtain a reaction solution; dissolving 100 parts of lignin in 200-300 parts of deionized water, then adding the mixture into the reaction solution, performing heating to 60-90DEG C, carrying out heat preservation reaction, and adjusting he pH of the product to neutral so as to obtain the lignin grafted polycarboxylate. The lignin grafted polycarboxylate has a carboxyl content up to 2.0-3.3mmol/g, which is close to the carboxylcontent of a high-efficiency polycarboxylate dispersant, and can be used as a high-efficiency dispersant in the field of pesticide preparations.