Charcoal is an important oxidation foaming agent in the preparation of inorganic open-cell foam wave-absorbing material. And it is a good wave-absorbing agent added to sample preparation process. The foam samples with different charcoal volume content such as 0.1%, 0.5%, 1.5%, 2% and 3% are prepared at laboratory. The dielectric constant of the five samples is measured and the reflectivity of the samples at X wave band is calculated on theory. Through test measurement, the conclusion is obtained that the best character is got when charcoal content is 3% during the preparation of inorganic open-cell foam wave-absorbing material, which provide reference for material preparation.