The invention provides a preparation method of a dye dispersant. The preparation method comprises stewing lignocellulose under the action of a catalyst to obtain cellulose and a black liquor, wherein the catalyst comprises one or more of sodium hydrosulphite, sodium hydrogen sulfite and sulfur dioxide, and carrying out membrane separation on the black liquor to obtain the dye dispersant. Lignocellulose is stewed by an acid sulfitation method under the action of a specific catalyst and the dye dispersant subjected to membrane separation has good staining performances and good heat-resistant stability. An experiment result shows that the dye dispersant obtained by the preparation method has fifth-level diffusivity at a normal temperature, heat-resistant stability (140 DEG C) of fifth level or higher, cotton cloth staining performance of more than forth level, terylene staining performance of more than third level and 40% solid content viscosity of 20-200mp.s.