Equation describing Brillouin backscattering are obtained in one-dimensional inhomogeneoua plasma. Based on the equations, the scattering spectra are calculated both for linear and nonlinear multimode instability. For thermal noise source, the spectral shape is dependent on the ion-acoustic wave damping, the plasma inhomogeneity and the incident light depletion. The resultant spectra including these affects are in agreement with experimental observations.