The invention relates to a preparation method of a dispersant with high efficiency and low formaldehyde, which is mainly used for assisting leather chemicals, i.e. a retanning agent and the like, to disperse in leathers when the leathers are processed and tanned. The method is characterized by comprising the following steps: (1) the synthetic reaction of a naphthalene sulfonic acid (A), i.e. firstly, adding naphthalene into a reaction kettle, and dripping a sulphuric acid into the reaction kettle to generate the naphthalene sulfonic acid (A); (2) the synthetic reaction of a phenolsulfonic acid (B), i.e. adding phenol into the reaction kettle, and dripping the sulphuric acid into the reaction kettle to generate the phenolsulfonic acid (B); (3) mixing the naphthalene sulfonic acid (A) and the phenolsulfonic acid (B), adding a right amount of water into the mixture for dilution, controlling the acidity of the mixed solution, then adding formaldehyde into the mixed solution, controlling the temperature of the mixed solution from 95 DEG C to 100 DEG C, preserving the temperature of the mixed solution for 8-9 hours, and then neutralizing the mixed solution to a pH value from 7.5 to 8.5 by NaOH; and (4) adding a formaldehyde trapping agent into the mixed solution so as to obtain an offwhite powdery dispersant finish product with high efficiency and low formaldehyde after drying processing, wherein the formaldehyde trapping agent is soyabean protein powder, isinglass, carbamide, melamine, quadrol, polyene and polyamine or diethylenetriamine. The dispersant with high efficiency and low formaldehyde has lighter colour and luster, strong dispersion force and little content of free formaldehyde.