The invention discloses a preparation method of an ester type high-dispersity and high-slump-retaining polycarboxylic acid water reducer. The preparation method comprises the following steps: (1) carrying out esterification reaction; (2) carrying out copolymerization reaction; (3) carrying out neutralization reaction. According to the preparation method disclosed by the invention, unsaturated acidanhydride and polyether diol are esterified to obtain an esterified product containing carboxyl and multi-ester groups; the ester groups in a molecular structure are gradually hydrolyzed under an alkaline condition of cement slurry to release the carboxyl with a water reducing effect, so that the slump-retaining performance is improved; the esterified product with an ester and ether copolymerizedtype side chain, an esterified monomer and isopentenol polyethylene glycol ether are copolymerized to form a polyester and polyether type side chain, a polyester type side chain and a polyether typeside chain which have different lengths; long and short side chains with different types are organically combined, so that the steric hindrance effect can be increased and the initial dispersion performance is improved; the ester type high-dispersity and high-slump-retaining polycarboxylic acid water reducer also has a good slump degree retaining capability and also has a certain air entraining effect.