This paper introduces the importance of surface wave attenuation rate of radar absorbing materials and its measuring method. Using the radar equation, this paper analyzes the influencing factors by the measurement of radar scattering cross section under near-field conditions. The antenna beam width and spacing between transmitting antenna and receiving antenna are important error sources of radar cross section measurement. A scheme is proposed to reduce the measurement error by increasing the 1 dB beam width of the direction diagram and reducing the distance between the transmitting antenna and the receiving antenna. The 8~12 GHz conical horn antenna is designed and manufactured. The antenna has good circular symmetry, and the 1 dB beam width reaches 22°. A near-field RCS measurement system is set up，validation experiments of the system is conducted with metal cube. Compared to theoretical values，the difference of the RCS curve peak values are less than 0.5 dB, and the difference of the RCS curve average values are less than 0.2 dB. It can been seen that the system has a relatively high measurement accuracy. Then the surface wave attenuation rate experiments are conducted with the near-field RCS measurement system, the difference of the measurement result between near-field RCS measurement system and compact range is 0.38 dB, so the measurement method of this paper can accurately measure surface wave attenuation rate under near-field conditions.