The invention belongs to the technical field of concrete additives, in particular relates to a water reducing agent, and more particularly a composite polycarboxylic acid water reducing agent. The composite polycarboxylic acid water reducing agent is characterized by being prepared from the following components in percentage by mass: 8-28.6 percent of polycarboxylic acid high-performance water reducing agent, 0.5-7.36 percent of inhibiting agent, 0.001-0.15 percent of functional aid, 0.01-0.65 percent of preservative and the balance of water, wherein the total mass percentage of the components is 100 percent. The composite polycarboxylic acid water reducing agent is applied to freshly mixed concrete. After a template coated with waste engine oil or rosinate soap is dismantled, the surface of hardened concrete is smooth, air bubbles are reduced remarkably, and the construction requirement is fully met. By adopting the composite polycarboxylic acid water reducing agent, the problem of a large quantity of air bubbles on the surface of concrete, produced after the use of demolding agents such as waste engine oil or rosinate soap and the like is solved.