The invention discloses a high-performance concrete maintenance agent and its preparation method. The preparation method comprises the following steps: 1, preparing a polymer emulsion from 50-98 parts by mass of a vinyl group contained monomer and 1-40 parts by mass of a carbonyl or aldehyde group contained vinyl monomer; 2, dissolving 1-20 parts by mass of a hydrazino group contained monomer in the polymer emulsion obtained in step 1; and 3, uniformly mixing 30-70wt% of the material obtained in step 2, 1-3wt% of a film forming assistant, 1-3wt% of a plasticizer, 1-2wt% of an antifoaming agent, 1-3wt% of a leveling agent, and the balance water to obtain the high-performance concrete maintenance agent. The high-performance concrete maintenance agent has the advantages of simple synthetic technology, easily controlled structure, fast film forming speed and high water retention rate, and can form a film within 2h and has a water retention rate of above 90% when the solid content is 20%.