The invention discloses a special superplasticizer used for concrete with high and ultrahigh strength and a preparation method thereof. The preparation method for the superplasticizer provided by theinvention comprises the following steps: initiating a solvent, a polymerizable monomer A, a chain transfer agent B, a polymerizable monomer I and an initiator to polymerize so as to obtain an intermediate solution; then adding polyether C, amino acid D and micromolecular peptide E, and simultaneously adding an auxiliary reaction reagent F so as to obtain an intermediate mixed solution; and addingthe intermediate mixed solution into a mixed solution of water and a phosphoric acid composition G which is dissolved in a solvent so as to obtain a solution of the superplasticizer. Compared with a commercially-available ordinary superplasticizer, the superplasticizer provided by the invention has the following advantages: when the superplasticizer is applied to high-strength concrete with a strength grade of C50 or above, the usage amount of the superplasticizer can be significantly reduced to achieve a same fluidity; the workability is significantly improved under same fluidity conditions;the inverted slump cylinder emptying time is shortened; the viscosity is reduced; and when the superplasticizer is applied to ultrahigh-strength concrete with a strength grade of C100 or above, the advantages of the superplasticizer are more prominent, and the usage amount and viscosity of the superplasticizer can be significantly reduced.