The invention provides a preparation method and application of a small molecular phosphoric acid group additive. The obtained small molecular phosphoric acid group additive has the characteristics oflow doping amount, good slump retaining performance under medium and low slump conditions, and excellent clay adaptability. The preparation method comprises the following steps: (1) performing a prepolymerization reaction on a monomer a with a phosphorous acid group and aldehyde c to form a prepolymer and to obtain a prepolymer mixture, wherein the degree of polymerization of the prepolymer is 5-30; and (2) in the d prepolymer mixture, adding a monomer b and aldehyde d, performing a condensation reaction for end blocking of the prepolymer, so as to obtain a small molecular phosphoric acid group dispersant having a weight-average molecular weight of 2000 to 10000, wherein the monomer a has at least two reaction sites capable of participating in the condensation reaction, the monomer b hasa polyether structure, a molecular weight of 400 to 2000, and a reaction site capable of participating in the condensation reaction, and the molar ratio of the monomer b to the monomer a is 1:(5-15).