Previous studies have shown that the technology of beam smoothing may effectively control parameter instabilities within the laser-plasma interaction, and greatly reduce stimulated Brillouin scattering (SBS) and stimulated Raman scattering (SRS). However, the recent experimental results on NIF revealed a much higher SBS and SRS than expected, one possible reason is due to the coherence between incident laser beams. In our research, two laser beams from Shenguang II facility are employed to irradiate an Au plate target, the energy and spectra of the backscattered light are measured in different coherent conditions. The results show that the driven-laser beams strongly interfere with each other, and usually the backscatter becomes strong gradually with the increase of coherent degree between the incident beams.