For microwave diffraction imaging

we use the WKB approximation to estimate the total field in the dielectric object. Compared to the Born approximation

the amplitude and phase of total field in the object are modified. The Fourier Diffraction Theorem that is quite similar to the one proposed by Wolf（1969） is then derived. By using the extended filtered backpropagation algorithm the object can be reconstructed from the spatial spectrum. The computer simulations show that the quality of image reconstructed by proposed scheme is superior to the one produced by Born approximation.