In order to measure the fraction of laser-entrance energy, which is carried out mainly through the measurement of the fraction of backscattering light, a full aperture backscattering station measurement system is developed on the Shenguang-Ⅲ prototype laser facility. The system collects backscattering light from the laser-plasma interactions by the focusing final lens and decreases the beam caliber by the off-axial parabolic mirror. Stimulated Brillouin and Raman backscattering light are separated by single-edge dichroic beam splitter. The backscattering light′s energy, history and spectra can be detected by the system. The energy fraction of backscattering light is measured with and without beam smoothing. The results indicate that the fraction of laser-entrance energy will increase and the energy fraction of backscattering light will decrease with beam smoothing.