The invention provides an ester-based polyether and a preparation method thereof, and applications in preparation of water reducing agents, wherein the ester-based polyether has the following structure, R1 is H or CH3, R2 and R3 are respectively selected from C1-C5 alkylidene, R2 is the same as or different from R3, R4 and R5 are respectively selected from H or CH3, m is 1-130, and n is 0-129. According to the embodiment of the present invention, the ester-based polyether can be used for the synthesis of polycarboxylic acid water reducing agents, wherein the synthesis conditions are mild and easy to control. The structure of the ester-based polyether is defined in the specification.