The invention provides a short-chain branch cross-linked water-retaining solvent layer controlled-release type polycarboxylic acid slump retaining agent and a preparation method thereof. The polycarboxylic acid slump retaining agent contains the following components in parts by weight except water: 320-325 parts of polyether macromonomers, 3.0-4.5 parts of sodium hypophosphite, 2.0-6.0 parts of phosphate, 0.5-3.5 parts of hydrogen peroxide, 1.0-2.0 parts of ammonium persulfate, 101.7-124.8 parts of an initiator, 115-160 parts of a small monomer solution and 10-15 parts of liquor of industrialsodium. The method comprises the following steps: 1, performing primary charging; 2, performing secondary charging; and 3, preparing the finished product. According to the method, a reducing agent containing hydroxyl, amido and phosphate, a carbodiimide cross-linking agent, phosphate and other components are adopted, anchoring ability and moisture conditions of the short solvent layer are enhanced, a lubricating effect and a hydrophilic state of cement-based binding material granules are improved, cohesiveness of a cement-based binding material mixture is increased, and the segregation and weeping probability of the mixture is reduced.