Provided is a synthesis process for the one-step production of a monomeric polyether for polycarboxylic acid water reducing agents, wherein the monomeric polyether is synthesized in one step by mixing an initator and a catalyst at a temperature and a pressure, and then introducing same into a reaction kettle together with an epoxide at a certain ratio for ring opening polymerization. The synthesis process of the present invention realizes continuous production without the need of first synthesizing a prepolymer and then synthesizing a macromolecular monomeric polyether step by step, thereby improving the production efficiency. By separating four links, i.e. displacement, polymerization, curing and neutralization, in conventional monomeric polyether production processes, the present invention more effectively controls each of the links and increases the utilization efficiency of the reaction kettle; in addition, the process is easy to control, the structure of the product is stable, and the retention of double-bonds is high.