The density and depth of grain pile directly affect the evaluation of silo storage. However, there is no report on the detection of mean density of grain pile. To make up for this gap, this paper establishes the relationship between density and dielectric constant through free space transmission method, offering a feasible way to detect the mean density of grain pile. Considering the popularity of ground penetration radar (GPR) in silo detection, the author derived the formula of maximum detection depth, based on the special environment of the silo, the detection quality factor of the radar, and the features of the scattering cross-section. After that, the proposed method was applied to compute the density and depth of grain piles in actual silos. Finally, our method was proved to be accurate through simulation and experiments. The research findings shed new light on the electromagnetic detection of the density and depth of grain pile in actual silos.