The invention provides a preparing method of a star structure polymer and application thereof. The method is simple in operation process, a reaction process is controllable and easy for realizing industrialization, the prepared star structure polymer serves as a cement dispersant, has higher initial dispersing capacity, is small in gradual loss of concrete slump, can obviously reduce viscosity of high strength concrete and is favorable for improving pumping performance. The method comprises the following steps: (1) performing an oxyalkylation reaction on specific polyhydric alcohol polyethylene glycol as a compound A, specific ester group-containing epoxide as a compound B and specific polyether segment-containing epoxide as a compound C, wherein a molar ratio of the compound B to the compound C is (1-5) to 1; (2), hydrolyzing the ester groups to obtain the star structure polymer with a weight-average molecular weight of 10000-20000.