The invention discloses a preparation method of a synthetic type polycarboxylic acid water reducer. The synthetic type polycarboxylic acid water reducer has a solid content of 40-50%, and is prepared from the following raw materials in parts by weight: 100 parts of unsaturated polyoxyethylene ether, 7-15 parts of unsaturated acid, 4-10 parts of unsaturated ester, 1-3 parts of an amide derivative, 0.5-2 parts of an oxidant, 0.2-1.2 parts of a chain transfer agent, 0.3-2 parts of a reducing agent, 0.1-1 part of a crosslinking agent, 0.01-0.1 part of ferrous sulfate heptahydrate, 5-10 parts of a commercially available mud-resistant auxiliary agent, 90-130 parts of deionized water and an appropriate amount of liquid caustic soda. The preparation method disclosed by the invention adopts a normal-temperature oxidation-reduction initiating system to synthesize the synthetic type polycarboxylic acid water reducer which not only has functions of water reduction and slump loss resistance, but also has functions of mud resistance and early strength, heating is not needed for reaction, the energy consumption is reduced, and the production cost is greatly reduced.