The invention discloses a preparation method of molding sand for gray iron casting, comprising the following three steps: preparing raw materials, preparing the molding sand, and mixing the molding sand. The preparation method has the advantages of simple process steps, easy operation, no use of coal dust, clean workshop, little pollution, and low cost and environmental protection by using straw powder, modified starch and other renewable raw materials. The prepared molding sand has small gas content, short drying time, good permeability and high baked tensile strength in casting, and can improve the releasability of the sand mold, further improving the fineness of casts.