The invention discloses a method for preparing a concrete liquid admixture by utilizing waste water from production of alcohol from molasses. The method comprises the following steps of: evaporating and concentrating the recovered waste water from the production of the alcohol from the molasses to form a concentrated solution with the alcohol concentration of 35-39%, doping a lignin sulfonate concrete water-reducing agent modifier according to the proportion of 0.2-0.4% of the weight of the concentrated solution, and adjusting the pH to 3.5 so as to get a concrete water-reducing agent. Tests show that the water-reducing rate can achieve 8-9%, the concrete strength is not reduced, and reinforcement corrosion has no correction influence; compared with the prior art, the using quantity of sodium hydroxide is saved, the energy consumption for evaporation and concentration of waste water can also be reduced, the concrete liquid admixture can be used in situ, drying is not required, and the concrete liquid admixture can be used as a water-reducing agent for ordinary concrete and a retarding agent for the concrete admixture, and can also be compounded with naphthalene-based and other high-efficiency water-reducing agents for being used for concrete pumping agents and used as a binding agent for nonferrous metal smelted mineral powder balls, refractory materials and ceramic products in the smelting industry; and in addition, the process is simple, the equipment investment is low, and the operation and the implementation are easy.