Abstract: Using the plane wave reflection coefficient formula and transmission line theory of electromagnetic wave, the single layer absorbing material reflection coefficient formula of electromagnetic parameters were given when electromagnetic wave is the oblique incidence. At the meanwhile, the group discussion and matching design with the grid method were carried out to discuss the relationship among the material, thickness, frequency and the reflectivity. The result of research shows that, for the appearance design of low-frequency materials, the thickness gradient design should be adopted according to the strong absorption characteristics at different frequency points.