The invention discloses a formula and a preparation method of a C20 ballast block based on sandy waste soil. The C20 ballast block comprises the following raw materials in parts by mass: 9-16 parts of cement, 58-73 parts of sandy waste soil, 6-10 parts of mineral powder, 1-4 parts of gypsum and 9-12 parts of water. The preparation method of the C20 ballast block based on the sandy waste soil of navigation projects comprises the steps of sufficiently and uniformly mixing the raw materials in parts by mass, then injecting the mixture into a preformed mold, performing grading molding under the working pressure of 20MPa, immediately demolding the formed test piece, and naturally curing with water in a room to obtain the sandy waste soil ballast block. The strength of the cuboid test piece of the sandy waste soil C20 ballast block prepared by adopting the method can meet the strength requirement of C20 concrete, and can substitute the concrete for preparing hydraulic materials such as ballast blocks. The sandy waste soil is reasonably utilized in the method, so that transfer, rolling and adverse environmental influence of the waste soil in channel treatment are reduced, and resources are saved.