The methods of measuring the terahertz backward scattering characteristics of the background materials which are irradiated by quasi- plane wave have a great influence on the measuring results. To reduce the influence of the fluctuation of continuons wave laser output on measuring results, the backward scattering of the wall is measured before the foam plastic boards. Meanwhile, to eliminate the impact of wall on measuring result, the relative backward scattering intensity of different foam plastic boards and different azimuth angles of the same material are given, which is based on the biggest backward scattering intensity. The experimental results show that the scattering measurement errors of the three kinds of foam plastic boards which are relative to the wall have a significant decrease and thus the measurement accuracy of the relative backward scattering intensity in the follow-up parts is increased.