In this paper, the approximation of optical Fourier transform in the cases of defocus is studied, and the relation of condition required for the defocus approximation of Fourier transform is derived. From this principle a method of using the defocus approximation is described for deblurring an electronic micrographic image, which makes the optical deconvolution process significantly simplified. The experimental results show that the quality of the image of an amorphous carbonic film is notably improved.