The stimulated Brillouin scattering process under the interaction of high intensity sub-ns pulse laser with plasmas has been investigated, with a larger extent of underde-nse plasma being produced in advance. The occurence of instability of both stimulated Brillouin backscattering and sidescattering can be confirmed simultaneously through careful measurements of the energy and spectrum of the backscattered light, as well as the angular distribution of the sidescattered light, especially its space anisotropy.