Abstract: The weathered sand is weathered from the granite in the San GOrge. Because of the different weathered degrees and the unstable particulate grade, it belongs to a kind of inhomogeneous material. The mathematical model for its maximum dry unit weight ρ_{d max} and the content ρ₅ of coarse particle is established through the compacting test on the weathered sand. The figure of the three-factors, namely, compactnessd D, in-situ dry density ρ_d and content ρ₅ of coarse particle is given. Through the engineering application in the past three years the good result has been obtained.