The invention relates to a carboxyl terminated dendritic polymer adsorption material and a preparation method thereof. The dendritic polymer has a network spherical three-dimensional structure, and can greatly improve the adsorption efficiency as an adsorption material. The preparation method includes: dissolving cyanuric chloride in acetone or tetrahydrofuran, adding the mixture into an acetone solution of diol to obtain a primary hydroxy terminated dendritic polymer crude product, conducting purification to obtain a primary hydroxy terminated dendritic polymer; taking cyanuric chloride as the core, adopting the primary hydroxy terminated dendritic polymer as the branching unit to prepare a secondary hydroxy terminated dendritic polymer crude product and a tertiary hydroxy terminated dendritic polymer crude product, and carrying out purification; and dispersing the hydroxy terminated dendritic polymer in water, adding maleic anhydride and p-toluenesulfonic acid to prepare a carboxyl terminated dendritic polymer crude product, and conducting purification to obtain the carboxyl terminated dendritic polymer adsorption material. The method provided by the invention has the characteristics of mild conditions, simple process and short production cycle. The product contains a lot of active carboxyl terminal groups, has strong adsorption performance, and is expected to be used in heavy metal ion and organic wastewater treatment and other fields.