The invention provides a dry-mixed mortar water retention thickener and a preparation method thereof. The thickener is prepared from, by mass, 45%-55% of aluminum-magnesium silicate, 4%-6% of sulfonate, 30%-40% of cellulose, 4%-6% of air entraining agent and 1.5%-3% of water reducer. The sum of the mass percents of the components is 100%. The preparation method includes the following technological steps that all the raw materials are weighed according to the mass percents, aluminum-magnesium silicate, sulfonate, cellulose, the air entraining agent and the water reducer are put in a mixer to be mixed to be uniform, the dry-mixed mortar water retention thickener is obtained, and finally the finished product is packaged. The water retention thickener is good in peaceability, construction operation of all procedures can be met in the using process, construction efficiency is improved, no salt separation is generated on the surface of mortar after the water retention thickener is used, the stirrer mortar has good peaceability, plastic reduction, seepage proofing and antifreezing performance, and the durability of a building is improved.