The invention discloses a clay reinforcing agent for casting and a preparation method thereof. The clay reinforcing agent for casting comprises, by weight, dispersible polymer emulsion, waste sulfiteliquor, Curdlan, polyacrylamide, syrup and modified bentonite. The dispersible polymer emulsion, the waste sulfite liquor, the Curdlan and the polyacrylamide are mixed, and the contents of the components are controlled within a certain range, and then the clay reinforcing agent for casting is obtained. Through the clay reinforcing agent for casting, the addition amount of clay can be greatly reduced, the strength of clay sand can be remarkably improved, the mechanical performance of the clay reinforcing agent for casting can be improved, and the application range of the clay reinforcing agentfor casting is wider. Besides, the clay reinforcing agent for casting further has the advantages of being low in production cost, free of pollution to environment and high in adjustability, and the gas forming amount is reduced.