Plasma stealth technology has many unique advantages, hence it has a promising application in the aviation and aerospace fields. The attenuation characteristics of vertical incidence of electromagnetic waves into unmagnetized plasmas with metal underlay are studied theoretically and experimentally in this paper. Regulations for the change of electromagnetic wave attenuation with plasma parameters are analyzed in theory using WKB method. A-large-area plasma slab is generated stably by inductively coupled discharge, and the reflectivity arch test system of plasma slab is set up. While the attenuation effects of electromagnetic wave in unmagnetized plasmas are studied experimentally. The electron density of plasma generated at different discharge powers is obtained by using the microwave phase and plasma spectrum diagnostic technique, ranging from 8.17× 109 to 7.61× 1010 cm-3. The plasma generated by inductively coupled plasma (ICP) has an effect on the attenuation of electromagnetic waves, and the experimental results accord well with the theoretical ones. Results show that increasing the plasma electron density and covering homogeneity can contribute to improving the attenuation effect of plasma on electromagnetic waves.