Based on the reactivity ratio of comonomers obtained from the experiment, the supplemental time and amount of acrylic acid (AA) were determined by controlling the conversion ratio and adding active monomer according to theoretical calculation and experimental determination, so that the monomer composition of AA and methyl allyl polyethylene glycol (MAPEG) remained unchanged in the copolymerization system, so as to ensure the consistency of the copolymer composition generated successively. In order to systematically study the accuracy and reliability of the controlled feeding process (CFP), the composition difference of the copolymers obtained in CFP and traditional feeding processes (TFP) was compared. The effect of controlling copolymer composition on dispersion performance was investigated. The results show that the average acid/ether ratio [n(AA)/n(MAPEG)] of polycarboxylate superplasticizer (PCE) copolymers prepared by CFP based on the reactivity are 2.79, 3.77, 4.82, respectively, which are basically consistent with the designed goal (3.0, 4.0, 5.0). PCE prepared by CFP have higher average main chain length, carboxyl molar fraction and macromonomer molar fraction, which increases the coverage area and adsorption layer thickness of PCE, thus improving the initial fluidity and fluidity retaining ability when used in cement paste.