The invention discloses an alkali-sludge water-retaining agent as well as a spray drying preparation method and application thereof. Dechlorination alkali sludge is prepared into powder by virtue of a spray drying process as a water-retaining agent, and the powder has a 0.30mm sieve residue rate of 10wt%. The inorganic powder water-retaining agent prepared by virtue of the preparation method has good water-retaining property and is economic, environmentally friendly and capable of improving the water-retaining property of mortar and the shrinkage of concrete.