The invention discloses a preparation method for water reducing agent used in superfine slag powder production. The preparation method comprises the following specific steps: adding 120-130 parts of melamine and 40-50 parts of monoaldehyde into a reaction kettle, adding sodium hydroxide to regulate pH (Potential of Hydrogen) to 8-9, and reacting for 90-100 minutes under a condition of the temperature of 70-80 DEG C; carrying out precipitation; drying precipitates, spraying solid reinforcement particles in a drying process, and adding into the reaction kettle; meanwhile, adding 50-60 parts of melamine and 20-30 parts of monoaldehyde into the reaction kettle, adding the sodium hydroxide to regulate pH to 8-9, and reacting for 20-30 minutes under the condition of the temperature of 70-80 DEG C; after reaction is finished, adding sodium sulfamate of which the weight is 1/3 weight of melamine, and continuously reacting for 30 minutes to obtain the water reducing agent. The solid reinforcement particles are colloidal when meeting water and are mutually effectively bonded, the solid reinforcement particles are solidified again after being cooled, the dispersion of the superfine slag powder is improved, and a bonding degree is effectively improved. Meanwhile, according to internal metal particles, intensity is greatly improved after the superfine slag powder is made into bricks.