The invention discloses an anti-scale agent and a preparation method thereof. The anti-scale agent is a terpolymer made by adopting raw materials of epoxy sodium succinate, acrylamide and sodium vinylsulfonate, wherein the mass ratio of epoxy sodium succinate to acrylamide and sodium vinylsulfonate is (0.7-1.5):(0.5-1.0):(0.8-1.1). The preparation method comprises the following steps: copolymerizing epoxy sodium succinate, acrylamide and sodium vinylsulfonate, diluting, and performing depressurized steaming drying, drying and the like, thereby obtaining the terpolymer anti-scale agent. The anti-scale agent is simple in equipment, simple and convenient to operate, low in production cost, small in environment pollution, excellent in anti-scale property and relatively good in corrosion retardancy function, the anti-scale rate of a product can be up to 90.8%, and the anti-scale problem of high-alkali high-solid water can be solved.