The invention discloses a slow-setting polycarboxylic acid water reducing agent and a preparation method thereof. The water reducing agent is prepared from polyether, ammonium persulfate, oxydol, acrylic acid, mercaptopropionic acid, caustic soda flakes, calcium nitrate, sodium nitrite, deionized water and softened water. By optimizing the proportion of the initiator, bonder and other raw materials, selecting chlorine-free deionized water, using higher-dispersibility softened water, simultaneously using the slow-setting polycarboxylic acid water reducing agent, softened water, calcium nitrate and sodium nitrite and optimizing the reaction parameters, the slow-setting polycarboxylic acid high-performance water reducing agent has the advantages in water reducing rate, bleeding rate, gas content, setting time difference/compression strength ratio, shrinkage ratio and other critical indexes. The slow-setting polycarboxylic acid high-performance water reducing agent has favorable use effect and obvious cost advantage, and is worthy of wide-range popularization.