The invention relates to a method for low-temperature synthesis of a polycarboxylate water reducer by using isoamyl alcohol polyether, belonging to the technical field of water reducer preparation. The method comprises the following steps: by adopting isoamyl alcohol polyether and acrylic acid as raw materials, firstly forming an isoamyl alcohol polyether solution by dissolving isoamyl alcohol polyether into deionized water under heating conditions, respectively dropwise adding an acrylic acid aqueous solution and a mixed solution of a mercaptan type water-soluble chain transfer agent and a free radical type initiating agent into the isoamyl alcohol polyether solution, adjusting the temperature of a system, then performing copolymerization, and adding an alkaline regulator to obtain a finished polycarboxylate water reducer. A technical scheme disclosed by the invention has the advantages of high monomer utilization rate, more effective components of the water reducer, low cost and the like.