The invention discloses a preparation method of a cyanide-free zinc-plating brightener special for a sylvite system. The preparation method comprises the following steps that (A) 13.6-27.2 g of organic aldehyde compound, 45.8-75.8 g of secondary solvents and 300-500 ml of water are added into a reaction vessel, then heating is conducted, and the temperature rises to 40-50 DEG C, 10.5-22.5 g of reductive drug solution is dropped, and the dropping time is 50-80 minutes; (B) after dropping of the reductive drug solution is completed, the temperature of the reaction vessel rises to 80-85 DEG C, and after heat preservation and stirring for reacting are conducted for 6-10 hours under the temperature of 80-85 DEG C, a first step reaction intermediate can be obtained; (C) then 12.4-24.6 g of phytic acid aqueous solution is slowly dropped into the first step reaction intermediate, and the dropping time is 30-40 minutes; and (D) after dropping is completed, the reaction temperature is 70-90 DEGC, constant temperature is kept for 3-4 hours, cooling and recovery are conducted, and the cyanide-free zinc-plating brightener is obtained. According to the brightener obtained through the preparation method of the cyanide-free zinc-plating brightener, the deposition rate of a plating can be increased stably, brittleness of the plating is low, the working temperature range is wide, environment pollution is very little, and the brightener is beneficial to environmental protection.