Abstract: The feature of the ground clutter in Wide Area Surveillance (WAS) mode of SAR/GMTI system is studied. Based on this feature, the clutter plus noise spectral density matrix of one Doppler cell can be estimated from another Doppler cells clutter plus noise spectral density matrix, and the corresponding transformation formula is also deduced. Then a new clutter plus noise spectral density matrix estimation method is proposed which estimates clutter plus noise spectral density matrix of the #em/em# th Doppler cell from both the #em/em#-1 th and the #em/em#+1th Doppler cell. The influences on the space-time processing performance from target contaminated samples can be greatly reduced when the new method combines with the eigenvector subspace projection method. The simulation results show the effectiveness of the proposed method.