Laser and plasma interaction (LPI) is the important part for the study of inertial confinement fusion (ICF). In order to have an investigation for the energy and spectrum of scattered light produced by laser-plasma interaction in Shen Guang-III prototype laser facility, a backscattered light diagnostic system was developed. This paper introduces the design and the damping factor of this system. The laser focus shape after using the coherent phase plate (CPP) has been analyzed. Experimental results with the energy and spectrum of backscattered light have been provided and compared after adding the CPP on the laser beam. It is found that the fraction of backscattered light can be reduced to below 5% under the laser condition 8×800 J after using the CPP. And the spectrum range with CPP is narrower than that without CPP on laser beam. These experimental data can improve the research on hohlraum energy.