The invention provides a preparation method of a slushing agent for concrete. The preparation method comprises the following steps in sequence: implementing the first reaction between maleic anhydride and hydroxypropyl methyl cellulose in glacial acetic acid under the catalysis of acetate, so as to obtain the hydroxypropyl methyl cellulose monomaleate; and carrying out free radical polymerization between methyl allyl polyoxyethylene ether, acrylic acid solution and hydroxypropyl methyl cellulose monomaleate under an oxidentive-reductive initiation system, so as to obtain the slushing agent for concrete. According to the slushing agent for concrete, prepared by the preparation method provided by the invention, the molecular structure contains the hydroxypropyl methyl cellulose molecular structure unit, so that the macromolecules of the slushing agent can be entangled with each other to form a net-shaped structure to wrap cement and water for effectively preventing the moisture in the mortar from evaporating, and as a result, the slump keeping property is good. The invention also provides the slushing agent for concrete, and the slushing agent for concrete has the structure as shown in formula (1).