The invention discloses a preparation method of a novel anti-clay phosphate superplasticizer. The preparation method comprises steps as follows: (1) polyamine A and epoxyalkane are subjected to a ringopening polymerization reaction, and a branched polyether polyamine intermediate I is obtained; (2) the branched polyether polyamine intermediate I and epoxy chloropropane are subjected to a reaction, and an epoxypropyl ammonium chloride intermediate is obtained; (3) the epoxypropyl ammonium chloride intermediate and polyamine B react, and a branched polyether polyamine intermediate II is obtained; (4) the branched polyether polyamine intermediate II is taken as a substrate, a catalyst C, phosphorous acid and aldehyde monomers are added, the mixture reacts, and the phosphate superplasticizeris obtained. The prepared superplasticizer solves the problem about adaptability of current water reducers and clay, also has excellent retarding effect and slump retaining performance and improves the adaptability of various sand aggregates; raw materials selected in the process are low in cost, the reaction efficiency is advantageously improved, the reaction time is shortened and large-scale industrial production is realized.