The invention relates to a sulfonated tannin extract and a preparation method thereof. The preparation method of the sulfonated tannin extract comprises the following steps of (1) mixing tannin extract powder with water and filtering to obtain a purified liquid; (2) adding a methylating agent to the purified liquid for methylation reaction to obtain a methylated solution; (3) adding a sulfonating agent to the methylated solution for sulfonation reaction to obtain a sulfonated solution; and (4) adding a metal salt complexing agent to the sulfonated solution for chelation reaction. According to the sulfonated tannin extract, a soluble matter in the tannin extract is extracted to participate into subsequent methylation reaction and sulfonation reaction in the purification process in the step (1), so that the subsequent reaction efficiency is improved, the dosage of the methylating agent and the sulfonating agent is reduced, the obtained sulfonated tannin extract is high in tannic acid content, the influence of an insoluble matter in the tannin extract on the compatibility effect of SMP resin is avoided and the stability of a quality evaluation test of the SMP resin is improved.