Compared and analyzed the surface impedance defined by the basic definition of surface wave and by the reflection coefficient of thin radar absorbing material (RAM) lay- ers backed with a perfect electric conductor (PEC) plate, the relationships between its surface wave attenuation property and its reflection characteristics can be founded. Equivalent surface impedance for a perfect electrical conductor (PEC) plate coated by a thin layer of RAM can be easily obtained based on the reflection coefficient for an infinite plate coated with same thin RAM layers, which can be regarded as a function for computing the attenuation of surface wave in this RAM layers. In this paper, two kinds of reality RAM for single layer coating and two layers coating will be considered for verifying this approach respectively. For two layers coating, the mono-static radar cross section (RCS) reduction properties of a slab coated with this RAM are discussed for evaluating the attenuation of surface wave because the equivalent medium theory is not appropriated under this condition. The proposed approach is effective and advantageous especially for thin multi-layers RAM coating.