The invention provides an additive for improving construction performance of wall leveling materials. The additive is composed of, by weight, 0.2-0.5 part of polyacrylamide, 30-50 parts of soft soap, 20-40 parts of liquid paraffin and 9.5-49.8 parts of water. The additive is prepared by the following method: (1) dissolving polyacrylamide in water, placing the solution in a high-speed disperse mixer and stirring at the rotating speed of 800-1,000 r/min for 15 min; and (2) adding soft soap and liquid paraffin, and continuously stirring for 20 min. The additive has advantages of reasonable formula and simple preparation. Cost of the additive is only 1/3 of cost of a powdery additive. With a small addition amount of the additive, smoothness, coating property and sag resistance of wall leveling materials such as putty cream or building glue, etc. can be greatly improved. In addition, strength after hardening is raised, and construction efficiency and wall strength are better enhanced.