The invention provides a preparation method and use of a micromolecule water reducer with delayed coagulation and slump retaining properties. The preparation method has the beneficial effects that a preparation method is strong in adjustability and high in cost performance, a preparation process is simple, the prepared micromolecule water reducer has excellent slump retaining property, delayed coagulation property and clay tolerance. The preparation method comprises the following steps: (1) carrying out amination reaction: carrying out C=C double bond Michael addition reaction by virtue of organic amine with a specific structure and unsaturated carboxylate with a specific structure, and purifying, so as to obtain an organic amine intermediate with an amino hydrogen and carboxylic ester group structure; (2) carrying out Mannich reaction by virtue of the organic amine intermediate, formaldehyde and phosphorous acid, and purifying, so as to obtain phosphorylated organic amine intermediate; and (3) carrying out condensation reaction by virtue of carboxyl of the phosphorylated organic amine intermediate and polyether with a specific structure and a terminal OH or a terminal NH2 so as toconvert carboxyl into an ester group or acylamino, and alkalizing for neutralization so as to obtain the micromolecule water reducer with the delayed coagulation and slump retaining properties.