The invention discloses a preparation method of a low hydration heat anti-crack polycarboxylic acid water reducer. The method comprises the following steps: (1) acylation esterification reaction; (2)monomer blending; (3) copolymerization reaction; (4) neutralization reaction. Water reducer molecule prepared through the method contains ester groups, phenyl groups and phosphate groups, and is applied to concrete, and the ester groups is hydrolyzed in the alkaline condition of concrete, gradually releases carboxy groups with a water reducing function and hydroxyl phosphenylic acid groups with ahydration heat reducing function, can effectively reduce hydration heat of concrete, and prolong the heat dissipation time, thereby being capable of effectively preventing the problem of cracking of concrete to a certain degree and improving the durability of concrete. Meanwhile, as a polyether side chain is connected with a main chain through an ester bond, a layer of water film can be formed onthe surface of concrete, the problem that water on the surface of concrete is too fast to lose is reduced, the shrinkage of concrete is effectively reduced, and the water reducer is beneficial to reduction of the cracking of concrete.