The invention discloses a structure and method for testing the compactness of a ballast bed based on a polymer material. The method comprises the steps of: filling the ballast bed with a polymer material mixed liquid to solidify the gravel ballast after the mixed liquid is formed by reaction; then measuring the volume of a ballast granule polymer wrapped in mixed liquid foams; and calculating thedensity of the ballast bed by mass conversion. The method can measure the compactness of the ballast bed without changing the original relative contact state of the ballast bed.