Periodic structure of stimulated Brillouin backscattering spectra have been observed in ex-periments of intense Nd glass laser irradiating planar targets. Analysis and calculation indicate that this period structure results from the interaction between the ion-acoustic wave and the scattering light wave, the former is produced by the stimulated forward scatting and the latter by the stimulated backward scatting or scattering in other directions. From the wavelength shifts and period of the spectra, the temperature or drift velocity of the coronal plasma and the direction of forward scattering can be estimated.