Disclosed in the present invention is a process for preparing a (meth)acrylic acid-based polymer having a stable quality and suitable as a dispersant for cement. That is, the present invention provides a process for preparing a (meth)acrylic acid-based polymer, comprising the steps of the stage 1 of introducing (meth)acrylic acid and a polyalkylene glycol monoalkyl ether at a molar ratio in the range of 3:l to 50:1, subjecting them to an esterification reaction in the presence of an acid catalyst and a polymerization inhibitor and deactivating the acid catalyst with an alkaline agent in order to obtain an esterification product containing a (meth)acrylate and (meth)acrylic acid residues and the stage 2 of copolymerizing the (meth)acrylate and the (meth)acrylic acid residues at a pH value in the range of 1.5 to 3.5.