The invention discloses a preparation method of a polycarboxylic acid water reducer with high workability. The preparation method comprises the following steps: mixing acrylic acid with active monomers with high workability, adding an oxidant and co-initiator into the mixture, dropwise adding a mixed solution of unsaturated monomers, chain transferring agent, reducing agent, and water at a low temperature; carrying out reactions to obtain segmented copolymer (intermediate), finally adding methoxy polyethylene glycol and a catalyst to carry out esterification and grafting, and finally neutralizing the products. Through molecular structure design, active monomers with good workability are introduced into polycarboxylic acid water reducer; low temperature polymerization is performed at first, then high temperature esterification is carried out, damage rate of double bonds can be reduced, copolymerization degree is high, and the produced water reducer has good workability and high water reducing rate. The industrial production technology is simple, the production period is largely reduced, and production process is green, environment-friendly, and pollution-free.