The invention provides an additive capable of improving the water-retention rate and the operation time of a wet-mixed masonry mortar, a plastering mortar or a floor mortar in the building field. The additive comprises a component A and a component B, wherein the component A is prepared from the following components in percentage by weight: 1 to 1.5 percent of water-retaining agent, 0.1 to 0.5 percent of air entraining agent, 2 to 5 percent of water reducing agent, 2 to 4 percent of early strength agent, 1 to 5 percent of coagulant, and the balance of water through mixing and stirring; the component B is prepared from the following components in percentage by weight: 5 to 20 percent of sodium gluconate, 2 to 10 percent of saccharose, 1 to 6 percent of borax,and the balance of water through mixing and stirring; the water-retaining agent is methyl hydroxyethyl cellulose ether with the viscosity being 10000 to 20000mpas; the air entraining agent is one or two of sodium alpha-olefin sulfonate or fatty alcohol-polyoxyethylene ether sodium sulfate. The additive provided by the invention is reasonable in formula and convenient to use, can enable the wet-mixed mortar to have a better water-retaining property at high temperature when being added into the wet-mixed mortar, and has better storage stability. The invention also provides a preparation method of the additive.