The present invention discloses a carboxyl group-containing modified amino sulfonate high efficiency water reducer preparation method comprising the following steps: sodium metabisulfite, sodium p-aminobenzene sulfonat, bisphenol-A and salicylic acid are successively added into water and dissolved with evenly stirring to form a uniform solution; the solution mixture is warmed to 70-80 DEG C, and an alkaline liquid is added into the reaction system for adjusting pH value to 10 to 11; a formaldehyde solution is slowly added dropwise into the solution, the formaldehyde dropping speed is controlled, and then the solution mixture is warmed to 90-95DEG C for reaction, after the completion of the reaction, urea is added to remove residual formaldehyde in the solution, and then the solution is cooled to room temperature; and then the solution mixture is cured in a reaction kettle to obtain a dark red liquid, which is a carboxyl group-containing modified amino sulfonate high efficiency water reducer with the solid content of 36%. The MAS high efficiency water reducer prepared by the method not only has the characteristics of small dosage and good water reducing performance of a traditional AS high efficiency water reducer, also has a lower cost of production. The bleeding rate of concrete doped with the MAS high efficiency water reducer is significantly reduced compared with the bleeding rate of concrete doped with the AS high efficiency water reducer.