The invention discloses an anti-cracking and shrinkage type polycarboxylic acid water-reducing agent and its preparation method, its weight-average molecular weight (Mw) is 10,000 to 100,000, and its structure is . The preparation method of the invention is: (1) using gallic acid as a raw material to introduce unsaturated double bond structure to prepare gallic acid derivative unsaturated small monomer, and then react it with carbonyl unsaturated acid and derivative monomer to obtain water reducing agent prepolymer; (2) Introducing hydrophilic polyether side chain on the gallic acid derivative of the prepolymer to prepare a high-performance shrinkable polycarboxylic acid water-reducing agent. The anti-cracking and shrinkage type polycarboxylic acid water-reducing agent prepared by the present invention has good stability and can be stored under various extreme weather conditions. It has good dispersion effect in concrete, excellent shrinkage effect, and can significantly control and mitigate the cracking caused by various types of shrinkage.