The invention discloses a long-acting stable slump-retaining water reducer for medium and low-slump concrete as well as a preparation method thereof. One end of the slump-retaining water reducer is apolyether side chain and the other end of the slump-retaining water reducer contains 1 to 5 carboxyl adsorbing groups. The preparation method comprises the following steps: performing reaction on methoxy polyethylene glycol and epoxy chloropropane under the catalyst and alkaline condition to obtain methoxy polyethylene glycol glycidyl ether; performing ring-opening reaction on an amine compound and methoxy polyethylene glycol glycidyl ether to obtain aminated methoxy polyether; performing Michael addition reaction on the aminated methoxy polyether and unsaturated carboxylic ester to obtain methoxy polyether amine carboxylic acid ester; and performing hydrolysis reaction on the methoxy polyether amine carboxylic acid ester to obtain the long-acting stable slump-retaining water reducer. Theslump-retaining water reducer provided by the invention has moderate water reducing rate, is non-sensitive to water use amount and has small initial slump fluctuation; long-time stable slump retentioncan be realized; and the raw materials are low in cost, the synthesis process is simple and efficient, the produciotn energy consumption is low and the preparation process has the industrialized prospect.