The invention discloses a preparation method of a cement dispersing agent with clay adaptability and viscosity reduction effect. The preparation method comprises the following steps: gradually performing addition polymerization reaction on terminal amino polyether and diglycidyl ether material based on the molar ratio of 1: 1; adding 2-5% by weight of terminal amino polyether for sealing the end after the reaction is finished so as to obtain an intermediate product; and then performing phosphate esterification on the intermediate product and phosphoric acid through a catalyst and a water-carrying agent to obtain the cement dispersing agent. The cement dispersing agent prepared by the method is high in clay adaptability, good in dispersing effect, and obvious in viscosity reduction effect;the selected raw materials are low in cost; the synthesizing process is simple and efficient; the energy consumption in production is low; and the preparation technology has a good industrial prospect.