A method for preparing a thinning ceramic additive using a landfill leachate, comprising the following steps: filtering the landfill leachate; adding aqueous alkali and regulating pH to 7.5-9; adding a coagulant, then stirring and mixing with a blender; taking precipitates to mix with water to prepare a solution, adding a sodium hydroxide solution for alkalization, and regulating the pH to 7-8.5; adding a sulfonating agent, and reacting under an environment of 80-100° C. for 2-4 h; adding acrylic acid and N,N′-methylene bisacrylamide to the solution, then slowly adding the initiator, stirring under the condition of 80-90° C. to react for 1-3.5 h, and after the reaction is completed, drying the solution to obtain a solid matter; crushing the solid matter, and screening through a screen of 16-24 meshes; and uniformly mixing the above screened particulate matter with the montmorillonite and an additive to prepare the thinning ceramic additive.