The invention discloses a polycarboxylic retarding high-performance water reducing agent used for preparing concrete of middle and low strength grade. According to the weight percentage of the effective solid content, the water reducing agent comprises 9-15% of low air-entrained mother liquor, 0.5-3.0% of retarder, 0.01-0.03% of foam stabilizer, 0.03-0.10% of air-entraining agent, 0.02-0.08% of thickener, 0.05-0.20% of activator and the balance of water. The preparation method comprises the following steps of: weighing all compositions by the weight percentage of each effective solid content, putting and slowly mixing the compositions in a reaction kettle for 10-30min, thus obtaining the concrete. By adjusting the proportion of all compositions, the polycarboxylic retarding high-performance water reducing agent can meet the usage requirements of the concrete of middle and low strength grade (C15-C40), has the advantages of wide doping range, high water reducing ratio, good retarding effect, low chlorine and alkali content, low concrete shrinkage ratio, greenness, environmental protection and the like, and can prepare the green and economic high-performance concrete.