The invention discloses a sodium lignosulfonate dye dispersant and a preparation method thereof. The preparation method prepares a sodium lignosulfonate dye dispersant product with low contamination by using papermaking waste liquid alkali lignin as a raw material through a series of chemical processes like a sulfomethylation reaction, a substitution reaction and an oxidation reaction. Compared with a prior art, the preparation method provided by the invention greatly reduces the usage amount of blocking reagents like epichlorohydrin and oxidizing agents like hydrogen peroxide, reduces the temperature and pressure of reactions, and reduces production cost and reaction time. In addition, a product prepared by using the preparation method provided by the invention has slightly light color, and has heat-resistant stability and staining properties with test effects achieving or better than the level of professional counterparts; and professional test results show that the sodium lignosulfonate dye dispersant provided by the invention has staining properties achieving 4 to 5 grades (cotton, polyester fiber and acrylic fiber), and also has heat-resistant stability up to 4 to 5 grades (at155 DEG C).