The invention discloses a preparation method of a multiple-adsorption-group polycarboxylic acid water reducing agent. The preparation method comprises the following steps: 1) preparation of diazonium salt polymer: carrying out redox free-radical polymerization reaction on an unsaturated aromatic amine, an unsaturated polyether large monomer, a reducer and an initiator at a certain temperature under the regulating action of a chain-transfer agent to obtain polyamino polymers with different molecular weights, adding excessive inorganic acid and nitrite, and reacting to obtain the diazonium salt polymer; and 2) preparation of multiple-adsorption-group polycarboxylic acid water reducing agent: carrying out reaction on the diazonium salt polymer and inorganic salt according to a certain proportion under the action of a catalyst to obtain the multiple-adsorption-group polycarboxylic acid water reducing agent. The temperature, monomer proportion and large monomer molecular weight are controlled to regulate the product performance, so the reaction is quick and efficient and is simple to operate. The polycarboxylic acid water reducing agent prepared by the method is a functional polycarboxylic acid high-efficiency water reducing agent which has stable product performance.